"It would be very helpful if there was a tutorial how to add any device to the IDE, well at least the 8 bit types."
Agreed.
Until then, you can look at how maniac bug created the '1284P files and follow the same for the 644A.
- copy the boards.txt entry, make a new section for the new chip with the smaller memory size
- confirm that avrdude.conf has an entry for '644A, if not copy the 1284P section and change the signature bytes and memory size
- pins_arduino.h exists for a couple of pin mappings. The Bobuino variant for cards like mine, where I tried maintain pinouts for shield compability, and the other variants which are more like Sanguino pinouts, where D0 starts at the corner of the chip.
Cross Roads Electronics D0, D1, for Serial. D11-12-12 for SPI.A4/A5 for I2C.
http://sanguino.cc/
Follow the same Variants formatting for file placement that maniacbug did.