Currently the answer is indeed "no".
So you would like to be able to replace the existing buildings and upgrades with custom one's, like replacing a coal power plant with a custom geothermal one?
Would you also like to replace the building graphics and if so, what tools/formats would you like to use?

Tools? I'm open to whatever is available. As far as modifications, I just want to tweak some numbers here and there like those on warehouse storage.

Ah, I see - I was thinking about the creation of completely new buildings and upgrades. But tweaking values is obviously also a thing.We will consider it.