The description for Sverrir Valgeirsson‘s free Filmtagger software says it all:
Filmtagger is an application for those of us that still like to shoot film but use the computer to manage our photo collection.
Filmtagger makes it easy to embed EXIF, XMP or IPTC information in the scanned images about camera used, film, ISO, lens, date and other tags.
The program will scan the selected catalog and set the appropriate tags in all jpg or tif images it can find (a backup file will be created).