Hi Phil,
your answer and my update crossed eachother, so here is what I want to do:
Read the tag 'datetimeoriginal' from Sigma-X3F files and insert it to the corresponding TIF and PSD files which are already converted but in those files the tag 'datetimeoriginal' got screwed up by some software I was playing with (no back-up available).
With Exiftool I can restore the correct value from the original RAW file into the converted files.
Thanks, Hans.