I use GPilotS to transfer waypoints, routes and tracks in BOTH directions between my pda and my gps unit. I have an etrex Legend and a Palm m125.
To answer the original question, there is no way I know of to use a pda in conjuntion with Mapsource maps.
There is an easy way to transfer maps from a PDA to your Garmin GPS: "Palm sendMap'
This is available as a freeware at : http://gps.chrisb.org/en/download.htm
All this requires a serial link. There is no need for a laptop as GPilotS and this software is all you need to manage it from a Palm to a Garmin.
NC