Re: importing bb_users into wp_users
As somewhat of a novice in the mysql world, all I really know is what works for me. However, I do think that your suggestion would work.
One concern would be that some wp-specific data would be missing from the bbp-only users. Again, I don’t exactly know what that would be.
I don’t know if there is a quick and easy way to modify the user IDs in WP, since they are going to be associated with posts, comments, and so on. That might be more trouble than it is worth. It might be easier to just create new users in WP and change their ID numbers before they are associated with anything else. Then, delete the “old” users one by one in WP, and each time make sure you set the “Attribute posts and links to:” option to the new, highly-numbered user it should correspond to. At least that way you could add the bbp users in without conflicts.
There may be a fancy, streamlined way to do this, but I don’t know what it is.