General :: How To Turn Stock ROM In Ready-to-install ROM

I am interested in turn the stock rom of my phone in a ready-to-install rom. I mean, roms usually are .zip files with scripts that writes the customized files to the system file, overwriting the old files. I want to know how to take the stock rom of my phone and turn it in a intallable .zip file, like a customized rom.

After a normal installing the phone would be like the phone with the original rom, without changes.

I am interested in this for make changes and costumizations in the stock rom for then reinstall it. Normally, rom porting is made by taking a compatible rom (like CM) and taking the wished to port rom and changing directories like /system, /data, etc... Am I right? Well, if a would take the original rom of my phone and would turn it in a installable .zip, I could make ports, right?