For etrex 20 & 30 (but not the 10)
see the compare guide:
(see custom maps compatible)
Garmin have free instructions on how to use Google Earth and your custom images (scanned topographic route maps) in this case and loading it to the gps device.
Add the Image as Google Image Overlay
Reference the Image
Save as Places
Send Your Custom Map to Your Device
Connect your Garmin outdoor handheld device to your computer with a
USB cable. In Google Earth on the left side of the screen, right click
on your custom map. Click Save Place As.
In the dialog box that appears, find the drive for your connected
handheld device and save³ your custom map in the /Garmin/CustomMaps/
directory (you may need to create a new "CustomMaps" directory if this
directory does not exist). Save the file in KMZ format.