Is this possible to add more chips available for Arduino software?
In general, Arduino works like an custom made IDE for GCC, so it could be great to work for example with Wiring (ATmega128).
Regards,
There are perhaps a couple dozen places where you will find #ifdef's that check the processor type buried in the targets/arduino directory. These will all have to be expanded for each processor type. They are essentially everything that accesses a register. Atmel doesn't keep the names, addresses, organization, or semantics of the registers constant across the product line.
The IDE itself would need some updates to add the menu entry and flash size constants.
If the processor is not yet supported by the avr-gcc then there would be more work.