You can unlock the DLC even if you don't own the base game. Here's how.
1. Login or create your free Games2gether account.
2. Go to the reward page and link your Steam account.
3. Go back to the reward page and click the button to redeem your DLC or game.
You can also add Endless Space Collection, Endless Space 2 - Community Challenge Add-on, and Endless Space 2 - Uthassum Rhamoezz Sophon Hero from previous giveaways.