BuddyPress.org

export your blog to the wordpress format then install mu and buddypress, your blog can then still be accessed by /blog, but obviously you will have to import it first,

most of the plugins will work with mu but some will not, i guess you can only hope and try them out

you will need to add your plugins again, i suggest you add them one by one to see what is working and what isnt, hope that helps, let me know if you need more help