some empires like the rebels or other should have the option to capture ships for their fleets
2 ideas
1:an event where you got a 50% chance to capture an enemy ship of unknown make
example: the rebellion has the chance to capture a ship from an imperial shipyard
if you select to capture the ship and succed you get that ship for your fleet
if you fail you must wait 2 years before trying again
2: during battles an event go pop up about one of the ships in the fleet are damaged and could be boarded and captured

If possible it you shouldnt be able to capture death stars or super star destroyers everything else fair game