I remember Daniel saying you could add an <effects> tag to a repair skill, but I don't know if that works yet. Either way, you can use an attack on your own units if you want. You'd just want to make <attack-strenght value="0"/> and give it beneficial effects. Of course, the units wouldn't know to use it, but that's Glest AI for you. You can also give the unit an attack that will do damage to the enemy and produce an effect on only allied units in the area.