Creating a custom rig in 3ds Max that can mirror is fairly complex. If you can find some of Paul Neale's old rigs Maxine or Maxwell you can see the code he uses to do it. You've got a couple of alternatives though. 1. Use Biped or CAT that come with 3ds Max. 2. When I use a rig that doesn't have mirroring I animate both…
I am actually working on a tool to create custom rig, and I'm stuck with this kind of problem too. I didn't find an answer yet but i'm still atrying. I have made a thread here: http://www.polycount.com/forum/showthread.php?t=143933 , maybe something will come up.