You can add maintenance modules to ships in v3.2. They cost 200 MC, are size 25 and require 125 crew. They are similar in principle to mining modules or terraforming modules. When a ship or a PDC containing maintenance modules is on, or in orbit of, a population, the maintenance modules are added to the total maintenance facilities of that population for the purposes of maintaining ships in orbit. However, unlike normal maintenance facilities, they cannot build maintenance supplies.
This will allow you to create a fleet base without needing a colonisable planet. You can designate any rocky body as a colony, stick a PDC on it with maintenance facilities and you have a base where ships can be maintained or overhauled (assuming they are small enough for the available facilities). You will need to either ship in some minerals, mine them on site, perhaps with automated mines, or send them in via mass driver from elsewhere in the system. If you want to resupply, you will also need to ship in some maintenance supplies (if you don't have any planetary maintenance facilities to build them).
Steve