Thanks Mark, that was a super hint 
With the addition above its possible to load a sketch into a ATmega2560, now i need the Bootloader for it.
When taking a look at the Lady Ada Bootloader page: Arduino Hacks
there is described how to modify and make a bootloader, next when looking at the Bootloader code: hardware\arduino\bootloaders\atmega*ATmegaBOOT_168.c*
There are a lot of places where the ATmega1280 is mentioned, if i would now just duplicate (or expand the if) them and adapt for the ATmega2560 i think it works :-?
Need to get a Mega and a ATmega2560 to try that