It wouldn't make sense to use that chip with external mosfets... If you want to use separate power mosfets on the output, use one of the Class-D modulator chips designed for that configuration. Low distortion class-D amplifiers are very sensitive to specific parameters of the mosfets used in the output stage (one issue being the switching times for the pair of P/N-channel mosfets, if these are different from what the modulator expects you will get distortion).
Here's a random example of a class-D modulator chip:
http://www.ti.com/product/tas5001This one is designed for a specific companion H-Bridge chip but I have seen ones designed for discrete mosfets before. You can see in the functional diagram for the TAS5111 (the companion chip) that it's performing "timing control" to avoid distortion / switch-over current drain:
http://www.ti.com/general/docs/datasheetdiagram.tsp?genericPartNumber=TAS5111A&diagramId=SLES111It also looks like the PAM8403 already has a bridged output stage, so reversing that and driving another set of mosfets from there would be pretty awkward...
Another option, if you just need a little bit more power, would be to use a different integrated IC, for example:
http://www.ti.com/product/tpa3123d2 Two of those in bridge-tied configuration should make for a decent enough 25W (into 8Ohm load, typical hifi speaker) amplifier...