The ONLY ways you can “legally” import external game assets is:
If you are the original creator of said assets, purchased the map or assets from a 3rd part creator/studio and thus have a license.
You have formal permission from the asset creator to use and the creator specifies where it can be used at. This should be more than “word of mouth” a written letter or email printed out will suffice.
If a asset is “royalty free” from a 3rd party website. Like 3dmodels from any of the free to download and use sites out there.
This mean NO MORE ripping and flipping assets from GTA5, Train SIm, COD, etc.
Making models is not THAT hard and it is more rewarding when you create you own props from your own brain for others to enjoy rather than stealing.