I suppose it's theoretically possible, however I've noticed a number of things that are inherently different just in the directory structure alone.
First thing on the top of my head is that in WP (Stand-alone) there is a cache folder. WPMU has built in cache (AFIK). In any event, there are /slight/ differences in the folder structure.
Secondly, would be the configuration itself. WP (Stand-alone) doesn't require any settings to distinguish between sub-domain vs the directory structure. Simply importing all your DB config options from WP to WPMU would be insufficient.
Sure, you can import just about everything and reasonably mirror your WPMU files to closely reflect your original WP installation. You will however encounter problems beyond what's mentioned above.
Your /new/ installation will not know how to handle new user registrations. Your installation will puke when it's asked what to do. The solution, add the appropriate DB tables. Easier said then done, because core files will have to be modified in order to tell your WPMU what do.
Key components will be missing, such as the unspecified amount of additional code tha has been added or subtracted from the WP code-base in order to make WPMU work.
Needless to say, it would be a very tedious task and not worth the effort. Even the .htaccess file would be different for ModRewrite.
The simple solution would have you install the WPMU code as usual. Your time would be better spent importing your posts and copying your theme files over. Lets not even mention your plugins...
Ultimately, a fresh installation of WPMU, in my minds eye, would save you valuable time. The time it would take to fix WP to make it work like WPMU far outweighs the time it would take to troubleshoot a WPMU installation with WP imports and comparison checks.
Thats just my humble opinion anyway.