How low can I make the source impedance of a PIC16F887 ADC?
You cannot damage it provided you don't exceed the pin voltage range, which is in this case from -0.3V to Vdd+0.3V. 

You can guarantee that with a Zener diode (connected with cathode to the pin and anode to ground, rated the same as your Vdd is), but as long as the source itself gets the power from the Vdd rail (opamp?), it is not necessary.

The device itself has got clamp diodes at each pin (google CMOS input protection) which clamp the voltage to the supply rails, but better not try that, you could damage the MCU and/or its surroundings easily.