Im sure that you cant unless you make a mod to do so.

Your only opetion to get rid of a unit is to deconstruct it, but that wont give you experience points.

This is where pirates help, they will level your ships up faster. But i assume that oyu know this already.

EDIT:

If you could attack your own structures, i cant see how that would br profitable at lower levels as it would be a lot cheaper to buy a level for your ship, then making structures to blow up.

At high levels where money is not an issue, i can see how this could be useful, but then why not just get a larger fleet. But this doesn't give you an answer to your question.

Even if you could target something you own, your caps wouldn't get xp for it. Only destroyed enemies give you any, and the xp system sadly isn't very modable.

Sorry yo be the bringer of bad messages, but i don't think eather is possibel. There is a way however. You can make an ability which spawns a frigate (note that you can make it look and act as a building, frigate is just the entity type) as an enemy, by making an enemy spawn it. It could work the way that an ability infests the enemy, and the frigate will be spawned when the enemy is destroyed. You could make it so that once the new frigate is destroyed it will automaticly spawn a new one (unlimited xp ) and give it a good amount of hull and shield or make it steal resources from you when it is killed.