[Imp]

The game advises you dont do this but it seems to work fine though it requires a bit of forward planning.
In a defend scenerio set it up as a meeting to start with & deploy the force you want to react early or set them as reinforcements then switch scenerio type to defend.
This stops those units being dug in & leaving fortifications, the other option is to deploy them so their now empty fortifications form a 2nd line of defence that the defenders could use. However remember these fortifications will become visible to the attacker once he sees the owning unit.
If you set up a counter attack it is also worth putting some arty as reinforcements.setting them so arrive in time to support the attack.