Not until GPSD is ported to the 2006 OS (is it?) and Maemo Mapper gets GPSD support (see Feature Request Tracker Item #8).

Edit: until then, there is at least one (rather ugly) workaround that I can think of (NOTE: I haven't tested this and I don't know if it will work. I'm not responsible for any damage this procedure may cause to you or your software/hardware):

1. Connect to the GPS receiver using rfcomm to, e.g., /dev/rfcomm5

2. Set up a "named pipe" called, e.g., "/tmp/my_pipe", by entering the following commands into an xterm (or create a script an run that):
The script should "hang", waiting for input.

3. Use "/tmp/my_pipe" as your "Receiver MAC" in Maemo Mapper, utilizing its "undocumented" rfcomm support.

4. When you're done with Maemo Mapper, you can rm kill the script with ctrl-C or "kill", and remove /tmp/my_pipe, or you can keep them both around, leaving the "cat"/"tee" running in the background until the next time you run Maemo Mapper. You can even run the above script at startup if you always want it to be available, and set up a cool naming scheme based on the time of day or the number of NMEA files you've created.
(Note: tee will probably "queue up" the NMEA data from /dev/rfcomm5 until something (in this case, Maemo Mapper) reads it, so it's probably not a good idea to leave the GPS receiver on and Maemo Mapper off at the same time for a long period of time. It won't wreck your system, but it might cause the tee to crash, meaning you'll have to restart the script.)