Pulled from another forum after a quick Google search:
BIOS is stored on a chip on the motherboard. You would have to know machine code plus have the manufacturing specs for every chip and circuit on the board, so in a nutshell, no, you can not create your own BIOS.
You might be able to do a small overclock using software, but everything I've read says its extremely difficult to get a stable system using software instead of BIOS.