Sounds like a bad digitizer (front glass piece) or the ribbon cable didn't get plugged into the mainboard properly. If you did the job yourself you should disassemble and make sure the ribbon cable from the digitizer is seated properly in the socket. If you had the repair done at a shop I would take it back and ask them to fix it properly. First thing though... Try a software restore after backing up your data and see if that cures the issue.
Short answer: It cannot be done Long answer: Adding a SIM slot has been done on a select few devices but is really only useful to read the data off the SIM. Your device has a CDMA radio and is not capable of communicating on the frequencies necessary for GSM. Quick fix: Buy a GSM version and sell your Sprint version.