I've seen questions that say they can be unlocked, but none say how to do it.
The Overbridge level was part of a holiday promotion where if you decorated x number of floors then the Overbridge was a prize. Same with Panna City Medicines and Imperial Marches.
The other 6 levels that can be unlocked, 2 each for Endor, Chiss and Kashyyyk, are done so by sending salvage droids to Endor, Chiss, or Kashyyyk when they show up where you can send your droids.
Every time the scanner runs each 12 hours, the 3 areas change and are random. If you are lucky, Endor, Chiss, or Kashyyyk may show up in those areas. Your chances of getting one of those levels as a prize increases by area type. If you send a droid to the 'Chance for rare' area, for example, you have less of a chance of getting a rare prize such as an unlockable level. Sending to 'Good chance for rare' and 'Best chance for rare' obviously raises your odds for getting a rare prize but the cost to send the droid goes up quite a bit.
I've actually been lucky enough to get 1 unlockable level from the lowest chance area, but that was the only rare prize I've ever received.