First, I've exported all IDs into Excel, then fill out new IDs (exactly bigger than can be in WPMU base - starting from 10000), then I searched for duplicate emails in wp_users and bb_users, exported to Excel and changed new_IDs of duplicated users to IDs_from_WPMU (something like that :).

Then i've created array $old=>$new

$change=array(
1=>10001,2=>10002,3=>10003,5=>10004,7=>10005,8=>10006,10=>10007,11=>10008,12=>10009,13=>10010,14=>10011,15=>10012,16=>10013,17=>10014,18=>10015,19=>10016,20=>10017,21=>10018,22=>10019,23=>10020,24=>10021,25=>10022,26=>10023,27=>10024,28=>10025,29=>10026,30=>10027,31=>10028,32=>10029,33=>10030,34=>10031,35=>10032,36=>10033,37=>10034,38=>10035,39=>10036,40=>10037,41=>10038,43=>10039,44=>10040,46=>10041,47=>10042,48=>10043,49=>10044,50=>10045,51=>10046,52=>10047,53=>10048,54=>10049,55=>10050,56=>10051,57=>10052,58=>10053,59=>10054,60=>10055,61=>10056,62=>10057,63=>10058,64=>10059,65=>10060,66=>10061,67=>10062,68=>10063,69=>10064,70=>10065,71=>10066,72=>10067,73=>10068,74=>10069,75=>10070,76=>10071,77=>10072,78=>10073,82=>10074,83=>10075,86=>10076,89=>10077,92=>10078,94=>10079,95=>10080,97=>10081,99=>10082,100=>10083,101=>10084,102=>10085,103=>10086,105=>10087,106=>10088,107=>10089,109=>10090,110=>10091,111=>10092,113=>10093,114=>10094,115=>10095,116=>10096,117=>10097,118=>10098,119=>10099,121=>10100,122=>10101,123=>10102,124=>10103,125=>10104,126=>10105,667=>10106,669=>10107,670=>10108,671=>10109,672=>10110,673=>10111,675=>10112,676=>10113,678=>10114,679=>10115,680=>10116,681=>10117,682=>10118,683=>10119,684=>10120,685=>10121,686=>10122,687=>10123,688=>10124,689=>10125,690=>10126,691=>10127,692=>10128,693=>10129,695=>10130,696=>10131,697=>10132,698=>10133,699=>10134,700=>10135,701=>10136,702=>10137,703=>10138,704=>10139,705=>10140,706=>10141,707=>10142,708=>10143,710=>10144,711=>10145,712=>10146,713=>10147,715=>10148,716=>10149,717=>10150,718=>10151,719=>10152,720=>10153,721=>10154,722=>10155,723=>10156,724=>10157,726=>10158,727=>10159,728=>10160,729=>10161,730=>10162,731=>10163,732=>10164,733=>10165,734=>10166,735=>10167,736=>10168,739=>10169,740=>10170,741=>10171,742=>10172,743=>10173,744=>10174,745=>10175,746=>10176,747=>10177,748=>10178,750=>10179,751=>10180,752=>10181,753=>10182,755=>10183,756=>10184,759=>10185,761=>10186,762=>10187,763=>10188,767=>10189,769=>10190,771=>10191,772=>10192,773=>10193,774=>10194,775=>10195,776=>10196,777=>10197,779=>10198,780=>10199,794=>10200,795=>10201,796=>10202,797=>10203,798=>10204,799=>10205,801=>10206,803=>10207,810=>10208,811=>10209,815=>10210,837=>10211,842=>10212,847=>10213,848=>10214,857=>10215,861=>10216,871=>10217,872=>10218,880=>10219,905=>10220,926=>10221,936=>10222,937=>10223,943=>10224,944=>10225,955=>10226,961=>10227,962=>10228,964=>10229,965=>10230,967=>10231,968=>10232,969=>10233,972=>10234,973=>10235,974=>10236,976=>10237,977=>10238,978=>10239,979=>10240,980=>10241,981=>10242,982=>10243,983=>10244,985=>10245,987=>10246,988=>10247,993=>10248,994=>10249,996=>10250,997=>10251,998=>10252,999=>10253,1000=>10254,1001=>10255,1002=>10256,1003=>10257,1008=>10258,1011=>10259,1018=>10260,1019=>10261,1021=>10262,1024=>10263,1061=>10264,1062=>10265,1063=>10266,1064=>10267,1065=>10268,1073=>10269,1074=>10270,1076=>10271,1077=>10272,1078=>10273,1090=>10274,1091=>10275,1092=>10276,1093=>10277,1094=>10278,1096=>10279,1097=>10280,1098=>10281,1099=>10282,1101=>10283,1102=>10284,1103=>10285,1104=>10286,1128=>10287,1129=>10288,1130=>10289,1131=>10290,1132=>10291,1133=>10292,1135=>10293,1136=>10294,1137=>10295,1138=>10296,1139=>10297,1140=>10298,1143=>10299,1144=>10300,1145=>10301,1212=>10302,1408=>10303,1410=>10304,1412=>10305,1413=>10306,1415=>10307,1416=>10308,1417=>10309,1418=>10310,1419=>10311,1420=>10312,1421=>10313,1422=>10314,1423=>10315,1424=>10316,1425=>10317,1426=>10318,1427=>10319,1428=>10320,1429=>10321,1430=>10322,1431=>10323,1432=>10324,1433=>10325,1434=>10326,1435=>10327,1436=>10328,1437=>10329,1438=>10330,1439=>10331,1440=>10332,1441=>10333,1442=>10334,1443=>10335,1444=>10336,1445=>10337,1446=>10338,1447=>10339,1448=>10340,1449=>10341,1450=>10342,
10001=>5,10002=>31,10050=>1095,10205=>22,10267=>23,10055=>2,10274=>40,10280=>32,10306=>15,10303=>1096,10314=>29,10322=>11,10327=>9,10325=>35,10341=>1092);

echo "
<pre>";
foreach($change as $old=>$new){
echo"
UPDATE bb_users SET ID=$new WHERE ID=$old;
UPDATE bb_usermeta SET user_id=$new WHERE user_id=$old;
UPDATE bb_topics SET topic_poster=$new WHERE topic_poster=$old;
UPDATE bb_topics SET topic_last_poster=$new WHERE topic_last_poster=$old;
UPDATE bb_term_relationships SET user_id=$new WHERE user_id=$old;
UPDATE bb_posts SET poster_id=$new WHERE poster_id =$old;
";

}
echo"</pre>
";

This script returns SQL code changing old_IDs to new_IDs everywhere (or everywhere I could find) in bb_* tables.

And, finally, export bb_users and import it to wp_users.

Note!
Use "Complete inserts" and REPLACE or UPDATE instead of INSERT, otherwise you will get 1) incomplete query (cause wp_users have 2-3 additional columns) 2)"duplicatу key" error

I just did something similar with a single install of wp.

Keep in mind that when you use wp for user data, it will use all the ids in the wp_users table. When you click the option to use that db for user data, it doesn't merge anything at all! So for example, if one user has an id of 5 in bbpress, when you use the wp db for user data whoever is 5 in that db suddenly gets the bbpress guy's data.

So! To get your data into shape, what you need to do is map the bbpress guys to the wp guys. So 5 in wp should be the same guy in bbpress. You might need to change the id of that user in the bbpress db.

Also! there is a bbpress user metadata table, so be sure to change the id there, too.

This is how I tackled it. There might be an easier way, or some tool, but if there is, I couldn't find it. I just did some SQL on the db directly to get it structured as I needed it, then turned on integration.

it's a great idea but you miss something: there are a lot of duplicate IDs and users :)

I have a forum running on bbPress and a WordPress MU, which I installed later. Both have users, some of them is present in both bases.

So, we have two bases with some duplicate IDs and users. How can I integrate bbPress users into a WPMU database with less price?

Thanks