Hey sry to bring this back up, but i have just got round to it. OK, so i can have mon files with 51 52 53 53 yes? aswell as the 1-25 ones there already.
I added the mon.wil and .wix to the data folder, made sure the name was mon52 and then added a new mob, using the imgindex of 51(-1) 2(-3)...