It's not simple at all.
A license is a contract between people of companies for the use of something.
If you create an original game you do not get it licensed as it is already your property. Maybe you mean get it published?
You can license the rights to use assets you have created and that you own the copyrights on to someone else - usually for a fee and for a specific purpose. That is usually the problem with porting assets from some other game. They are owned by someone else who has already licensed their use to someone else for a specific purpose - that purpose being the game they are used in and no other.
And if you own the rights to a game, you can license or assign the assets or the entire game to someone else, either free or for a fee. Usually if you publish a game through a game publisher, you will assign some of the rights to the publisher in return for publishing it.
If you create an original game (NOT a mod of some other game), with your own original work and without any assets copyrighted by someone else. Then you own that game and become the copyright owner of all of it's assets. In that case, you can do whatever you want with it. Including allowing those assets to be used by others. By licensing the game or assets you are giving the licensee the rights to use those assets in only the way you specify in the license. You get to say what that license allows.
If you are the creator, and no one else is involved, then you can port - or allow your property to be ported to any game that allows
Here is where it gets complicated.
If you use the Bethesda Construction set or GECK or most other mod making programs to make that game, then you automatically assign the rights to use those assets over to Bethesda (no license required because it is included in their EULA) or the owner of that mod making program, and they can use those assets without asking or paying you. The EULA you agreed to when you downloaded the game making program is a contract and it states what can and cannot be done with mods or games made using it.
And you cannot license that game or any of the assets made using the Bethesda game making programs to any one else or use that game to make any money. For complete details on this, read the EULA you agreed to when you downloaded the GECK or construction set.
If you use some other game making program (called a UDK) to make your game, then that UDK will have it's own user agreement which may be more or less restrictive than Bethesda's.
I hope that helps.
