I dont think you can change the format in the way you suggest, and if you could the sentances would not then be in standard NMEA format.
HABAXE reads both GPGGA and GPRMC from a Ublox 6, then reconstitutes the various fields into this format for transmission as RTTY;
$$HABAXE1,sequence,time,lat,long,alt,speed,track,GPSruntime,Fchanges,batterymV,temperature,GPSlock,
flags*CHECKSUM,CR,LF
To ensure the HABITAT system decodes it correctly, for the circumstances when speed and track are null, I put a single zero in those fields ",0,". And to cut down on payload length, some fields are stripped of the characters after the DP.
The payload part;
sequence,time,lat,long,alt,speed,track,GPSruntime,Fchanges,batterymV,temperature,GPSlock,flags
Is sent as RFM22 data telemetry, and a remote reciever then reconstitutes the data into GPGGA and GPRMC sentances which allows the receiver to act as a GPS and feed live tracking information into mapping applications.