I am working on my custom story, and I got a key, and this key should open another door, in another level.
If this even possible, and if how do I do so? Please help!
The same way you would open it in the same level. The name of the key stays in your inventory, so you could have a key found in map A that would open a door in map Z.
Just make sure you place the callback in the .hps file associated with the map where the door you want to unlock is. If you are not sure how to unlock doors in the first place, check
this page and look for the AddUseItemCallback function to be placed in the OnStart() portion of your code. Then just write a function which will unlock the door, remove the key from inventory, and play an unlock sound if you choose when it is called.