I finally found it:
http://www.romhacking.net/utilities/536/
This is what other people used to map FFT. I figured I could do the same only the program wants .gns files and I don't know how to get them out of the rom.
They say .gns files are supposed to be located in the Map folder on the CD but you're talking about a ROM. Are PSX disc supposed to be readable like ordinary data CDs?
If you're using a ROM you'll probably need to mount the image with a virtual drive, then you can open it the same as if you had the disc.