sew to jef conversion is essentially conversion or export between two Janome file types, most like for some kind of design transfer purposes. You should be able to do this with Janome Customizer 11000 program as it works with most, if not all Janome formats. But it is likely many other embroidery software should be capable of this.
Open SEW file Open JEF fileAn embroidery designs manager
A converter for embroidery designes
Advanced version of the popular Embroidermodder
A home embroidery software
Embroidery software for Janome sewing machines
An embroidery design tool from Janome
A free embroidery editor and converter
An embroidery editor and converter
An easy embroidery and apparel decoration design software