How it works the ps2 ELF modding/compilling?

[Mirakichi]

Hi, I would like to know as is done the moddings on ps2 elf games? e.g; the RE4 Cheats Edition with a custom menu on the beginning with manys options, and GTA San Andreas with all cheats unlocked.
If I change/add a routine in the "nop" areas in the elf using the ps2dis, I can save the elf modified only in the .pis format, but the ps2 don't run the .pis format, only the elf.
So, how to convert .pis to .elf to run the modified on the console?

[NERV Agent]

It's been a few days and no one has answered your question, so I guess people here aren't too familiar with that.

However, I think these people might better answer your question:

http://psx-scene.com/