The method of uploading to MapSource is identical to loading to the unit.
THe files should be in either CSV or GPX format. sound (MP3) files and icons (BMP) should be in the same directory as the file. You Directory can be organized with sub-directories and they will be grouped that way as well.
I found I had to keep adding extra POIs to my Main Directory and then re-add the entire directory to the device otherwise just loading a single category overwrites the entire Custom POI directory on the device. So I have one folder I store POIs in and then categorize them how I want in it and upload as a big batch.