Can this be done???

I've been modding Medieval Total War for quite a while now and have made a lot of minor changes, I wanted to make the following change but am guessing it's not going to be possible?
When you build a certain building (e.g. town watch), a unit is automatically created (e.g. urban militia) in that province. In this way I would change the game so that it would only be possible to have urban militia by building a town watch.
Do all units have to be trained or is it possible to have it this way? (I was then thinking of extending this, so when the royal court is built then you automatically receive a unit of knights.)
I know it sounds pretty strange but anyone think it would somehow be possible to do? ~:confused: