It turns out that you can do this under Kiln. When you view the commit, you can view the cases it's linked to. This allows you to add a case. I assumed that you would do it from fogbugz, but evidently, all information goes from kiln to fogbugz not vice versa...
I'm going to leave this open in case anyone has a way to actually do this from fogbugz, just for completeness.