Certain embroidery design programs can import TrueType fonts and use them to create various embroidery texts and save them them in their own embroidery format. Your best for JEF embroideries is to use Janome professional software.
Convert TrueType font file to Janome embroidery layout.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert ttf to jef on Windows
An embroidery design tool from Janome
Convert from ttf 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.