There are some ways...
You can use Orb of Lightning (New) to cast, as a passive ability, any active-targetable ability, even with a cooldown. Just customize the ability fields and replace the "Purge" with the ability you want. The downside of this is that it's an Orb Effect (will collide with other orb effects) and it's only activeable via direct attack orders; doesn't work with automatic combat engage.
Another way would be with some triggering, like Barathrum bash, which is a dummy passive ability (Does nothing but display data, the icon, and have a level), but works entirely with triggers.
Example: To make searing arrow a passive ability, add Orb of Lightning and set the cast chance to 100% and the ability to cast = Searing Arrows.