Migrate from WP to WPMU

Hi guys,
this is not a plugin related question, but as i'm not familiar with mu i'm here to ask

I have a WP installation heavy customized (with new post types, taxonomies, terms, custom user profiles, and so on) all via coding (on template, no plugins). This site is online and running with hundreds of users that are collaborators.

is it possibile to transform a wordpress installation from normal WP (always updated to last version) to WPMU? and (as I know that everythig is possibile

Do you have any tip or alert on doing something like that?

if it was a new installation i would have no problem testing, but as it's full of users... you know, I don't want to mess upp everything