Geoget contains basic set of exports only.
While you need to export into some special format, like a GPX
optimalized for some GPX unit, you must use one of additional exports.
For Magellan see this one:
http://geoget.ararat.cz/doku.php/user:skript:gpxmagellan
(on the left side under menu you can turn on Google-translate). On the
page is contact for plugin author too. If you have a problem, you can
try to contact him by an e-mail.
Another way is to create export byself. Exports are done by an
extrernal scripts. However some basic programing skill is required.
This was a first stage of a problem - export. Second stage of a
problem is transfer into unit. You can made it by your favourite tool,
some GPSbabel, or something else. You can call your transfer tool from
script too, if you wish. (see the excellent Combine addon)
note: what informations are transferred into unit - it is fully
depending on how capable is your GPS unit, and how is written export
script.