I don't believe you can. The either/or audio mic settings are "Front" (the on-camera stereo mic) or "Rear" (inputs via the rear XLR connectors). There isn't an allowance or setting for using both front and rear at the same time.
If you want an on-camera mic and another external one, best bet would be to take off the factory front stereo mic, and replace it with another one in that mount (mono) that can be connected to the XLR inputs at the rear. You then still have another rear XLR input available for the additional mic you want to use.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com