It is if you have a copy of Nero - at least version 4(?) onwards. In the Nero installation folder is a file called neromp4.flt which you have to make a copy of, and put into your Audition program folder. When you restart Audition, you'll find it in your file types list when you open or save anything. Works fine...
No such thing in Nero 7 or 8 - I guess they've changed or removed the plugin architecture to stop you doing this sort of thing. I got the .flt file from elsewhere, but it just complains that it can't find the Nero Plugins directory.
I'll see if I've got any older copies of Nero around still to experiment with.
Paul