Working with OVL files

[nordlicht]

It is a military used GIS.

Hmmm. That as well as the file extension OVL reminds me of EADS's GeoGrid GIS, part of which is used in civil land survey, too. GeoGrid OVL files first existed as binary and ASCII types, afaik EADS added an XML type OVL eventually. GeoGrid should currently be at version 7, and as of version 4 there was a plugin named "Navigation" which could export and import GPX files. So, should my guesswork be correct you might want to have a look if this plugin exists.