Interesting question. Have you tried installing one to see what happens?
When I run MJ to make an AVI file, when I click "rip" MJ presents a box that lets me choose which video codec I want to use, and configure that codec. Then I have to chose the audio codec.
When I added a new audio codec, (after MJ install), it just appeared as an option on the list.
I did have some probs when I upgraded my DivX codec from 5.03 to 5.05 (MJ didn't seem to know I'd upgraded?), but I can't remember any details ....