i believe IMEI is hardcoded into the hardware, maybe on some chip/flash or something. i'm far from expert but i can tell that since complete reflashing on any kind of phone dosen't change IMEI, its not in the OS or any config file available on the phone.

You need to be technical experienced with electronics, since you need to read/write a certain flash rom inside the phone. As far as i know you need special cable and special software to do this.

Your best bet is to research this topic on some phone hacking websites.. wich i can not recommend, most public hacking sites are filled with virus's, trojans.and other malware.. kinda ironic.