If you are trying to load your Sketch/Software on to the ATmega8 then you can compile the Sketch using the different board setting that uses the ATmega8 Controller and burn it using the Following Fuse Bit's.
ATmega8: Use these fuse-bits and lock-bit.
hfuse = CA
lfuse = DF
unlock = 3F
lock = 0F
and you cannot burn bootloader's swapping them for other chips because the Siganture is Different and i'm not sure that will be accepted by the avr-gcc compiler.
Regards,
Nishant Sood