Trigger, AI to use custom skills?

For example if you have say downloaded custom skills "non JASS or trigger" Just custom modified skills... Can you still trigger these skills via "unit: order targeting a specific unit or point"

How can this be done? Does it have anything to do with the Text - Order String - Use/On value in abilities?

If the ability/spell is activated normally (i.e you have an ability which you can cast) then yes, just use the spell's order to cast the spell and it will work fine.