No, not with the limits of the RTW engine and EB's recruitment system.

The best way to get around this is to not retrain, but instead combine units. If you end up with remainders that are less than half strength, you can build a fort behind your lines to hold them. This way you can constantly cycle your reinforcements, particularly if you're in a particularly grueling defensive position that sees battle every turn.

Beyond this, what the historians would tell you is that the whole point of EB is to represent historicity as much as we possibly can. It would not make sense for the Romans to retrain hastati in Carthage because that never actually happened. It encourages you to think strategically - can you create a large enough army to accomplish your conquest quickly? Are you able to send a reserve without overextending yourself? Can you recruit local auxiliaries or hire local mercs to hold your objectives, as they are easy to replenish locally?