How can you make new srb2doom builder commands?

Can someone help me how can I make doom builder commands to make new fofs?

It's possible, but you need to have advanced coding knowledge to do so. You need to create a new linedef type in SRB2's source code, as well as incorporating it into Doom Builder itself. That's hrder than it sounds, believe me. I've attempted a few EXE modifications myself, and it took me a few hours just to figure out which code file controls what. On top of that, I still haven't been able to get them compiled.

You can use the Custom FOF special to make special FOFs.