Is it possible to change excisting country borders?
This isn't possible with any of the tools on this site. However, you can do this using the in-built "Nudge" tool in the game.
does that mean i can import a custom made map into this mod?
The site does not have much capability, as far as countries go. The country creator tool is still in beta, as it is still lacking a lot of features.
Map/state editing/importing may become part of the tool, but I don't have any plans for that in the near future.