Find where it makes the $authorExtra variable - prolly in ubb_lib_posting.cgi is my semi-educated guess. Right before that subroutine closes, declare a new variable called $authorExtra2 or something, and put the post count into that var. Then call $AE2 from the template.
I have pizza in my hand, otherwise I'd be a bit more specific... if you look around in that sub, you can prolly get a good idea of how vars are declared and how to add stuff to them. If you need more detailed instructions, lemme know and I'll go fish around for it when I'm not eating.

---Skorpion