Janome Digitizer program should be able to convert between JEF and JAN embroidery files. If not, then there is probably no other way how to perform JEF to JAN conversion.
Convert Janome embroidery layout to Janome Digitizer embroidery design.
We found one software available in the database for this conversion.
Microsoft Windows software - convert jef to jan on Windows
An embroidery design tool from Janome
Convert from jef 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.