not sure where to make the modifications in the library
Modify the library? What library?
I can send the equivalent
nemaNMEA messages for decoding which is not a problem. What I want to do is send the messages back using two xbee pro's.
Ok... you know I can answer questions about NeoGPS, not XBee, right?
It looks to me like the ublox GPS devices can receive RTCM messages (I do not know the format of these messages). The ublox device will use the corrections in those RTCM messages to slightly alter the data sent back to the Arduino in the standard NMEA messages (e.g., GGA). NeoGPS does not need to be modified to parse these "corrected" fields in the standard sentences.
You're going to have to be much more specific, because I can't see where you need NeoGPS help.