I don't consider retraining as cheating. In fact, sending new troops from your homeland into the war is easier because your main armies almost never have to stop their conquest.
My way is having a half stack (Led by a young fm) behind the army as "reserve" and merge. When the reserve stack is empty, I form a new one in the homelands and send it to the front. I don't like the micromanagement of smaller armies of single units, so I gather first and march afterwards. Also, the command of the reserve is a good task for promising fms that have yet to learn about real war and logistics when they finish academy.