Is it possible to allow for a user to push new GIt repos without having to manually add them to the gitosis.conf every time?
According to this blog post, gitosis does not support wildcard repositories. The author does however link to a patched version of gitosis (here) that supposedly provides this functionality. I have not used it.
I'm not sure if this will be of any help to you, but it's worth noting that the "successor" to gitosis, gitolite, has built-in support for wildcard repositories (documentation here).