You could wait the integration of bp in wp,
or (i think it’s better) you could try copying some tables in your new mu install:
example:
copy wp_users to wp_users
wp_postmeta to wp_1_postmeta
wp_posts to wp_1_posts
etc etc..
that’s a bit difficult to do, maybe others could help you better than me.. 
Nicola
You should ask johnjamesjacoby about this migrate issue. He’s done something like that recently. I think the answer is you need to export your data, using phpmyadmin or with the native export/import tool in wp.
http://www.techtraction.com/2007/07/30/move-or-backup-wordpress-data-with-the-built-in-importexport-feature/
Lots of stuff if you google ‘wordpress export’, ‘wordpress import’
Alessandro, I’m a bit confused with what you are trying to do. It sounds like you have a normal (single) WP installation and are looking to go social with it. A lot has happened since you posted this and BuddyPress is now available for single WP installs. I’m testing it out now. In WP v3.0, WordPress MU will be merged with the single WP installation and there will be a migration path. I can help answer questions around this… just let me know some more specifics.
@timnicholson
Why revive an 11-month thread?
r-a-y, I definitely understand that these days people like to consider older posts and forum threads dead after a short period of time. Some user just like to jump on old stuff without finding the answers in newer threads. However, when there is a dearth of information about a particular topic and people are trying to properly research before posting and all that comes up is old threads, I’m of the opinion that they should be resurrected. Why start a new post asking the same questions and diverging the threads? I’m actively searching for information about this and scrounging through PHP code and websites for answers. I’ve uncovered some good information that I’m trying to share, but am still looking for more answers. If I run a search and it returns all these “dead” threads and no new threads, the masses are going to as well.
Plus, I just replied to other threads offering to help with questions you’ve asked here months ago to which I see no answers. If you’ve figured all this out, why not update those original threads with the answers for newer folks trying to figure things out? If you haven’t figured everything out, then why not be happy that the threads are going active again and information is being shared?
Please don’t take this the wrong way, I just don’t understand why some forums seem to be filled with people complaining about “old” threads being responded to and others appreciate it. And why don’t the forums that are dead-set against replying to old threads, simply close out the old ones?
I appreciate your rationale and enthusiasm, Tim.
It’s because certain issues might not be relevant anymore.
And I doubt Alessandro is reading this thread.
If anyone stumbles on this thread, there is another thread where I have replied at length about this: https://buddypress.org/forums/topic/activating-existing-users
“In WP v3.0, WordPress MU will be merged with the single WP installation and there will be a migration path.”
Unless you are merging together more than one standalone WP install into one multisite network, in 3.0 there will be no need to migrate anything to turn on multiple blogs (ie; multiple sites).
