This has probably been asked before, but is it possible to make siege maps?
Yes, it is If you want to make one go here for more info on the entities to use http://levelforge.teamhuh.com/forgelink/viewtopic.php?t=95 And you can also open the siege_hoth map for more info.
Are there any available for download anywhere?
Not yet, but they will come Just look around on the forums, and you'll see some topics about the maps.
Siege maps are indeed able to be made and this has been answered before, but anyway there are already a few siege maps currently in development. You just need to look around the forums, I know that Nkenobi is making a siege map, not sure how that is going since my time away.