GPSBabel is a free software tool that can be used to convert different track file formats. It can be downloaded from gpsbabel.org.

Convert to different file formats
The following command converts a logger igc file into a OziExplorer plt file. Please note, that 2 files are written. output.plt is the file with barometric altitude while output-1.plt uses GPS (above MSL) altitude.
gpsbabel -t -i igc -f -o ozi -F output.plt

The above example generated OziExplorer plt files. GPSBabel supports many track file formats.

A list of all supported formats is listed on the GPSBabel homepage.

QNH correction
The logger stores the barometric altitude with QNH=1013.25. In order to adjust for the real altitude the barometric altitude must be corrected using the following formula:

This can also be done with GPSBabel:
gpsbabel -t -i ozi -f output.plt -x height,add=[dalt] -o ozi -F output-2.plt
with [dalt] beeing delta alt as calculated in the first line of the formula above. Of course the above line can be called with different file formats as well.