you can generally tweak the tuning coil of a radio to receive frequencies that are close by to the original tuning range . for example, stretching out the coil of a broadcast band fm radio to receive the civil aircraft band 118mhz above,though the former is fm and the latter transmissions are AM,you can still here the transmissions.
but the GMRS radio like you have works on the UHF band that is 450-470mhz
so practically there is no way to tweak it to receive the fm broadcast band.
for increasing the range all you have to do is make an rf power amp and fix a relay to trigger it on when ur walkie talkie is transmitting.
cheers