To write EXIF data for my adapted Konica Hexanon 40mm f/1.8 lens I used this command:
exiftool -FNumber="2.8" -MaxApertureValue="1.8" -FocalLength=40 -Lens="Konica Hexanon 40mm f/1.8" PHOTO.JPG
For cameras of other brand you might want to use
-ApertureValue="2.8" instead of (or in conjunction to)
A backup will be made with name PHOTO.JPG_original.
Interestingly the size of edited photo is smaller, so I wonder if some details are lost (I don’t think so, but I haven’t figured out yet).
When I copied the modified JPG back to the card, the camera read the focal length correctly, but f/2.8 appeared as f/2.4… (edit: MaxApertureValue uses APEX values not F-Stops. c.f. http://photo.stackexchange.com/questions/19143/how-can-the-aperture-value-written-in-exif-be-larger-than-the-nominal-limit-of-t)
(Added Feb 15, 2016: To copy tags from one photo to another — http://thomer.com/howtos/copy_exif.html)