In the improvements editor in CivIII you can make a building "alllow veteran ground units" (or "air" or "naval"). But this is not all it does. It also allows the upgrading and fast healing of units. And it is impossible not to allow all three of them if you just want one. I know it is not very good for the gameplay to have three different buildings in the standard epic game. But in a simple mod (perhaps a middle age-only scenario) it would be cool to build Barrakcs (elite units), Armories (upgrade units) and Infirmeries (heal units).