I was recently looking at the Skaven Grey Seer and noticed that it has only 2 spell types, thus I began to hatch an idea. Please correct me if I am wrong. The Skaven a mad scientists and inventors, so if the captured a Empire Wizard, genetically modified a Grey Seer with its power, then you have a Grey Seer with 3 spell type in a way. Please note that this is just an idea.

What if, though, in battle a Grey Seer pulls off The Dreaded Thirteenth on an Empire Wizard and his unit. Does the wizard become a stupid, mindless slave rat or could be possibly be captured and teach his secrets to the Skaven under the pain of torture.

The great horned one doesn’t allow any magic but its own, so the only way of geting other spells is threatening or Bribeing a Mage of another race (or maybe brainwashing) to cast them for you.
Other Clans wouldn’t like that though and since that Wizard Lacks the Paranoia and Reflexes of a Skave he’d probaply be Assasinated real soon.