Nope, not really. I believe it works that the game will not allow more than a full stack under your control at any given time. So, enough reinforcements will enter to accomplish that, but after that, any remaining will be unavailable. I know it's not the greatest thing, but if you end up in this situation, you may be best off letting the AI control the reinforcements for that battle, since they should enter and be AI controlled with no problem at all - it's just that the UI only allows the human player to control X number of units which causes the problem.