gpsbabel supports a vast variety of file formats which, once converted to .gpx or .kml, can easily be displayed by Marble. Many people do not know about gpsbabel though, and using it forces you to use the command line and know things about file formats which many people are not comfortable to look into. - detect whether gpsbabel is installed on the system (available in $PATH) - if yes, extend the File => Open dialog for additional extensions - if such a file is selected, use gpsbabel to convert it to a temporary file and open that with Marble
See http://www.gpsbabel.org/capabilities.html for a list of file formats gpsbabel supports