Yet Another Multi-site Question

I know this question has been asked a million times, I just want to make sure that I get this right,
I have an existing non-multi-site that has been in production for some time. I am not using BuddyPress, I am using Mingle on this specific site for membership etc.
I want to add wiki and ask functionality to the site, but the current theme is not going to handle rendering wiki or ask very nicely, and I don't want to change the main theme.
I want to make sure that the logins stay the same, in other words, if they goto the wiki area or ask area they will have the exact credentials as they had at the main site and vice-versa.
I don't want to wreck the main site by moving it to a multisite.
Is all this possible?