Added two DLC to the base for the game Hidden Through Time, but it is possible to add them only as games. Please mark as game.
How can I add DLC in the future?
I was also unable to add a developer. Bug.
Sorry for my english, Google Translate helped me 
Unfortunately DLC can only be marked from the admin side. Just let us know and we’ll mark it. I believe you can add DLC through Giant Bomb as well and it will push through to Grouvee.
If I add DLC to Giant Bomb, they will instantly appear in the database Grouvee or will I have to wait?
It takes time to push over. @peter would know how often imports occur, but you want to give it some time to appear.
If you added it to Giant Bomb, they would take less than 8 hours to get over on Grouvee once it’s approved over there. Their approval process takes a day typically.
I’m going to add the ability to add DLC to Grouvee soon. My roadmap is roughly to finish the new shelf pages, redo the profile pages, then work on the database stuff. It needs some serious reworking, and we need our own database probably sooner than later.
Like @BMO said though, we can mark them as DLC on the admin side, just tell us what to do.
Thanks for the detailed answer.
And thanks for Grouvee. This is the best video game database I’ve come across so far.