And if yes how to do it best? What is the standard procedure here?
Should I include base package authors as contributors in DESCRIPTION???
Am I allowed to use MIT + file license with that or is it wrong to do so?
No, you must use the GPL, since the code you copied is licensed under
the GPL. You can choose to use version 2 or 3 (or both). You do not
have permission to re-license R code under a different license.
I think it's slightly murkier than that - MIT is GPL compatible, so
you could license the code you've written as MIT, but the package as a
whole would need to be GPL-2/3.
Hadley