Re: buddypress doesn't work on stand alone install??!!
No you can not do it in that way.You need to follow the process for migrating from wordpress to wordpress mu.
1.Export your posts/pages
2.take a backup of your wp-content directory
3.Take a backup of database(in case you need it to revert back)
4.remove the wp from server and install wpmu
5.Import the posts from your export file
6.Upload the uploads ,themes directory to wp-content/ in your new installation
If you have a website with thousands of posts,then export import may be trouble.you may follow this instructions in that case
Hopefully it helps.