I don't think the computer hires any neutral heroes. Ever. You need to force it to do it manually, if you know how to do it. I think you need to move a unit near the tavern and use a JASS command to hire a custom hero. Additionally, you would have to update the AI script to actually use that hero. Not easy at all.
Such things are a lot easier to do with AMAI Developer's Edition as AMAI knows how to hire neutral heroes (although there is only support for one tavern in it, more taverns would require code changes).
