After searching last night, I have finally found the answer:
http://i-nz.net/2009/01/15/selective-import-of-svn-branches-into-a-gitgit-svn-repository/
It seems that you have to actually go in and manually edit the .git/config file in order to add an svn branch to an existing git repo. So according to these instructions I would have to add an entry for each branch.