In the Init line of the leader of the group of spare troops write
extratroops = group this
Make sure that the player has a higher rank than any of the spare troops.
For the following I assume you want reinforcements when you have no soldiers left alive under the player's command. If that's not the case you can raise the number, but remember that there can be no more than 12 in a group.
Create a trigger and enter as the condition
(count units player) < 2
and for the On Activation line
{[_x] join player} forEach units extratroops
As written above, the trigger will happen when all the soldiers have been reported killed (someone says "Oh no. x is down.") and they get removed from the Command bar at the bottom of the screen.