Both (.jan) and (.jef) embroidery design files are variants of Janoma embroidery design and layouts and as such your best option would be to use Janome software, such as the Digitizer Pro. Although, it is possible that other programs may also be able to perform this embroidery conversion.
Convert Janome Digitizer embroidery design to Janome embroidery layout.
We found 2 software records in the database related to .jan to .jef file format conversion.
Microsoft Windows software - convert jan to jef on Windows
An embroidery design tool from Janome
Convert from jan file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned jan to jef. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert jan to jef on OS X
Converter for embroidery designes