There is no support inherent in the Eartmate to output NMEA strings. There is a $45 hardware device that will convert the 'Rockwell Binary Protocol' data to NMEA.

There has been some work on software libraries to allow using the Earthmate and other Rockwell Binary GPS hardware with MS-Windows instead of the hardware solution, but this would require linking the shim libraries into Netstumbler.

The best solution is to use a 'real' NMEA capable GPS. I'm shopping for one right now, haven't had much luck finding a good price (< $280) on the 'hockey puck' style RS-232 12-channel units.