They have, the SDK 
That means “Software Development Kit” and it’s what allows players and 3rd parties to add content to the game. As far as I know there’s no way for your average user to get their changes into the official game right now, but you can share it with the community and anyone interested can download it.
This thread will walk you through setting up the SDK. I haven’t used it yet, so I can’t give any tips for that.
I would bet that you can import models created in Sketchup into the game using the SDK. You might have to export it to a certain format, or find a separate program that will convert it to the correct format. My local small airport has no buildings; I’m planning on trying to model some of the buildings and put them in the game with the SDK.
Good luck!