Many map and navigation formats can be either export or converted to some exchange format or even between each other, hence it is likely that some programs may offer the possibility of conversion between Google Earth Keyhole format (.kml) and Garmin map files (.jnx). For example the MAPC2MAPC utility seems to be able to perform this conversion.
Convert Google Earth Keyhole Markup language file to Garmin raster map.
We found one software available in the database for this conversion.
Microsoft Windows software - convert kml to jnx on Windows
A digital map conversion software
Convert from kml file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.