Non-Passive? You mean that you want it like... Storm Bolt let's say? Well... It is pretty impossible since Bash activates only when the unit attacks and if you cast it as a normal ability, it doesn't take account of the damage. You can't really detect the damage of the unit, or atleast I didn't find out by now.
I think he means like searing arrows. Well this would have to be a dummy skill, based off of searing arrows. While active, it would give him a chnace bash and while inactive, not. And using JASS, you can simply trigger a dummy unit to bash the target who took damage, but you'd have to ask someone else for that.