Moving from PIC16 to PIC18 Adding chip to gpasm and picp

After getting a nice tool chain working on linux using gnupic's utils, 
sometimes the SDCC and picp with a USB programmer on my laptop I've got to 
move to the PIC18 chips and I seem to have run into a brick wall.

gpasm don't support the chip I want to use so first question is how do I add a 
processor to the list of supported chips?

The last time I checked SDCC didn't support PIC18 but that might have changed 
I'll go check that but picp don't support the chip I'm hoping to use so how do 
I add to that as well.

Perhaps this is not the correct forum for such questions but please direct me 
if I have made a mistake.