Hello:
Is it possible to use an FET or some other device to switch a single adc signal source, externally to the PIC, rather than by using another adc input pin?
If you're asking if you can connect multiple analogue sources to a single ADC pin through a multiplexor... then yes you can. You can use analogue switches, or analogue multiplexer ICs (e.g. 4066, 4051, etc.). If you're asking something else, then you can switch an analogue source using a relay or analogue switch.