Is it possible to return to Skuldafn?

It is possible to get to Skuldafn/Sovengarde after the main quest by using coc skuldafn02 in the console

You cannot get back there without console commands. The console command you are looking for is "coc " and you'll probably need to do it again to get out.

Use coc skuldafn02 to get there. The codes for Skuldafn are skuldafn01 trough skuldafn04. 02 should put your back in the Skuldafn temple. coc whiterun might bring you back home. Not sure about the last one.

Several mods exist on various platforms that allow one to return to Skuldafn. How it's done in game varies from mod-to-mod.

For Skyrim Special Edition on Nexus we have Fly to Skuldafn by candoran2 which allows one to ride a dragon (for example Odahviing) back.

The description for this mod states:

This mod allows you to return to Skuldafn on the back of any tamed dragon, Odahviing or Durnehviir once you've finished the main questline. Also allows you to get the unique dragon priest staff or return to Sovngarde.

On Bethesda Net, we have a mod called Return to Skuldafn by Darth_Mew which is available for: PC, XBox One, and PS4. (Yes, that's right, a mod that's available for the PS4.) This one works by enabling it as a fast travel location. The description for this mod states:

This mod allows you to return to Skuldafn and the ancient portal that resides there, the portal to the Nordic afterlife.