Op-Amp difference amplifier low-pass filter
How can I extend this circuit to a 1st or 2nd order low pass filter,
  with a corner frequency at about 20kHz.


1st order solutions: -

Put the same capacitors across R6 and R8 is one method (maintaining R6 and R8 at the same value).

Another method is put a 1k resistor in series with the output and shunt the 10k with a capacitor (it works more perfectly with the 10k omitted of course).



A 2nd order circuit can be done in a balanced input configuration but I'd use a sallen key op-amp filter after your circuit.