How can I change/enter the picture coordinates in the EXIF extention for Mapillary

I have a number of nice pictures that I would like to add to Mapillary and hence to OSM. Unfortunately, I forgot to switch the GPS on so I don't have a GPX-track. What is the easiest way to do this? I am not interested in changing any of the other picture attributes. If it could be done within JOSM I would like that!