I would LOVE to have this feature however I fear it may be unlikely... FPV Freerider is built in the Unity game engine, and unless the author built out a system to import maps, all maps need to be present at the time the game is compiled for distribution.
That said, I can think of a few ways you could create a basic 'Map Editor' by passing a grid of characters each symbolizing a different asset or element on the map. Importing Sketchup files sounds scary. There are a few guides on how to move individual models from Sketchup over to Unity, however importing an entire map of 'geometry' sounds like a big task. If you wanted to import individual assets, maybe you could have a special character designated for 'external imports' which would reference a full path to said object to import.
I'll keep thinking about ways the community could contribute to the maps featured in FPV Freerider because that is my #1 desire after playing both titles. The existing maps are amazing, but I love the idea of building my own.