Are there reinforcements and withdrawals of specific units?

[MrClock]

For example, is it possible to create a defense mission with few units, waiting for reinforcements?

[Perturabo]

ORIGINAL: MrClock

For example, is it possible to create a defense mission with few units, waiting for reinforcements?

There's no mechanic for reinforcements. The only way to do it is to create a mission on a somewhat larger map where the whole force is divided into defending unit and reinforcements and lock them into place when setting up the mission.
It's actually done in one of the official missions.