Is this new content inter-playable with my old content? Meaning...can I travel to that new land (Dragonborn dlc) and then travel back to continue my previous quests (Dawnguard). Or am I stuck with the new dlc until it is complete?

Thanks

You can come and go as you please.

You can freely travel back and forth between Solstheim and Skyrim.

Some quests will require you to finish them or escape in order to get back to the mainland, but they'll be pretty obvious.