There are 5 ability slots: 0-4.  The problem is the UI uses the 5th slot for a universal button on all capital ships, but that can be overcome if you don't mind rearranging the UI.  It's a whole new project, though.  Frigates can have 5 abilities easily.

As long as the 5th ability is passive, it doesn't matter, because you will never need to click on it, and the capital ship button always goes on top. So basically, what I'm trying to say is that if it's passive, you don't need to mess with the UI.

okay, it complains- but does it choke? as in, minidump? Cos I might be able to make things work then

It'll work as long as the ability in that 5th slot is a passive ability.  You won't have any way of activating that ability slot otherwise.

ApplyBuffToTargetsInRadius

OnDeath

     - DoDamage (for damage, obviously)

     - PlayAttachedEffect (for shockwave/explosion visuals)

Check out the Telekinetic Push ability, adjust the targets and trigger, and then just add an additional Instant Action for the damage effect.