To start off you have to put 2 projectile units with the same negative ID into the unit you choose. To do this, place an arrow on the map, change it to an archer in AoKTS, then go back into the editor and place 2 more arrows on the map. The result should be that 2 units have the same negative ID and you can delete the other one. When these 2 units are garrisoned into another unit, you can unload them indefinitely. Play around with this.
Now make an object in area trigger for the projectile you put in your unit. The effect can be whatever you like, +attack, heal, anything! Make sure to task (patrol effect for projectiles) the projectile away from the area so the trigger doesn't keep going off.
To heal a unit back to full no matter its current hp, refer to this article