Yes, the Kashyyyk, Endor and Chiss levels appear to only be available through using the Salvage droids. Considering that they added a new level and there is not a seasonal icon in the corner (as there was for the holiday theme and service to the empire), I'm not sure this is a limited time special event.
You can keep rescanning as much as you want (or can pay for) which will change which locations are available to send salvage droids to. One very useful tip: if a salvage droid is currently on a mission to a location when you rescan, that location will not change. For example, say you have Kashyyyk in the middle (15 bux) slot, and you send a salvage droid there and then hit rescan while it is on its mission - in this case, Kashyyyk will stay in the middle slot, and only the other two locations will reshuffle. Another tip: One location can be in multiple slots at the same time; for example Endor can be in all 3 (free/5+ bux, 15 bux, 25 bux) slots at the same time.