You can't do it at render time. You have to it after, with the help of ACDSee or RnameIt (that you can find on this site). Perhaps there is a way to do it with the Flipbook, however I don't know about that.
Also, there might be a way to do it with rendering scripts, but I'm clueless in this area so I can't help you, sorry!
Hope this helps
Salutations - Cheers
Bernard Lebel