My site is up- can I still create a Multi-site?

I've read quite a bit about the advantages of multi site, but I'm intimidated by the perceived process and not entirely sure if it's what I need. I think I want to have users from the main site, post mobile photos to a separate folder/install, which could be displayed back on the main site.
Why not just post to main site? membership is limited, restricted to subscriber, and even if I change their permissions to Author, or Editor, they still can not create a post. Only Admins can. It's a theme attribute. So got any ideas? I would like to download the mobile apps, but don't want to spend all that time, if this is not going to work.