Take a look at " '/profle.php' #2937 -- do=editavatar", and template 'modifyavatarbit'.
You can use the sections of code above to begin to create an additional avatar entry.
Sorry, I cannot provide you with a simple answer. Maybe there is a similiar MOD that you can use as an outline. I did a quick search, but did not come up with anything useful. I rarely look at the modification section, so I am not the best person to answer my own, "is there a similiar MOD" question. There are smarter folks out there.