BIOS customization is usually done by companies what developed BIOS (American Megatrends, Insyde, Phoenix, ...). So you tell them what you need and they will give you back binary. More expensive option is to buy the source code (you will not get the full source, but you will be able to add your own code e.g. to configure and initialize some peripherals).
BIOS development is very expensive may start around 20 000 - 30 000USD.
Other option could be a free BIOS e.g. have a look at coreboot:
https://en.wikipedia.org/wiki/Coreboot You can compile it by yourself and even test it. It is not difficult, just be sure you know what you are doing as if something is wrong, your PC may not boot up.