I think you are confused. Depending on exactly what you meant, this is easily possible.
You just want to modify the resources for a particular civs region. correct?
Just go into worldbuilder, scroll around until you find that area/civ and then place resources. Its really easy, I build entire mod/scenario's basically only using simple XML, worldbuilder, and modifying the WBS with notepad++.
This will obviously reveal the map to yourself, but once you go out of worldbuilder, it will be black again, and no other players would have seen it (the AI's).
I'd recommend taking a look at Dales' "An in-depth look at the WBS file"
its really quite easy once you get the hang of it.
Shiggs