You could do most of it in theory by making several copies of the existing units with different training times. You'd then edit the buildings file and make each of the recruitment buildings produce a different version of the unit, depending on the level of the building. Ie. Have 4 versions of hastati: Muster Field level barracks builds Hastasti 4 (4 turns to train), Militia Barracks builds Hastasti 3 (3 turns to train) etc.

It would be time consuming to set up, and there may be other implications. Personally I think it would a bit extreme to limit recruitment that way, but I suppose it has some merits - like slowing down initial expanse and the AI not stripping its settlements of people at the start.