Using GPSU (
www.gpsu.co.uk) you can read an Excel CSV formated file and then upload the data into your GPS. If you want the information in Mapsource you can then download the data from your GPS. This process isn't bad as it sounds if you only need to do it a few times. There are probably more elegant ways but it works for me. I've used this methodolgy with a Garmin 182C, 162 and a Legend successfully.
This software also has other features that may be of interest like editing/combining/spliting routes. Check it out. I've used it for 4 years with great success.