You can do this either through the script command AddObjectCreatures, or by clicking on the garrison, finding the ArmySlots parameter in the left window of the editor, right-clicking on it, in the pop-up window, clicking Add, then clicking on the plus sign that appears next to it, selecting the parameter from the list, and in the Creature field, specify CREATURE_STOUT_DEFENDER (this is the ID for the Warriors), and in the Count field, specify the desired number.