You can store heroes this way, but this is not how the game was designed so this was not extensively bug tested.
At least one bug has been reported that can lose you all your finished heroes.
If you are hoarding finished heroes in training camp “Alpha”, 
NEVER cancel training in training camp “Alpha”. If you must cancel training, first remove 
ALL finished heroes from training camp “Alpha”, then, and ONLY then, cancel the remaining training.
If this bug report is accurate, the game assumes that canceling any crafting/ training means you have collected all the finished battle items/ heroes, so it resets type of crafting/ training, amount of crafting/ training, and resources used for crafting/ training, and finished items/ heroes to zero.