It probably depends on the unit. I know that it is possible with the Magellan Road Mate 700 and 800 series since I have owned both. BUT, you also need the software "Magellan POI File Editor" to send the files to the unit (via the USB cable).
This software is Copyrighted so you will have a hard time finding it if you don't already have it. It is on the TOOLS CD that with the Unit. You can TRY to call Magellan but their track record for helping acquire this file has not been great.