I think there should be a fick so you could hire replacements for fsllen soldiers in a mercenary unit.

The thing is it seems like Mercenaries are a one battle then finish with them unit no matter what experience they get because you can't retrain them, which is a shame becauseit is fun to supplement your army with the locals, so when a mercenary unit is available for hire you should have the option of hiring or retraining the unit.