Hi,
Well, the information on this was somewhat confusing. Thus:
1-In the main Artemis window, after importing a .cif file to do a Feff calculation, the Atoms tab includes in the Export button the possibility of generating a .inp file with feff8 format; this is mentioned as well in the corresponding part of the Help documentation. Since no warning appears at that point about the impossibility of using that .inp file in Demeter, one would believe that such thing is indeed possible.
For this reason, although indeed no feff8 executable comes with Demeter (my excuses for having assumed otherwise), I managed to obtain a feff85L.exe file and put it some time ago in the same Demeter directory as feff6.exe.
Then I forgot that feff85L.exe did not come with Demeter. And when choosing the feff executable with the Edit preferences menu item, I thought that the mention there that feff8 was not yet supported was outdated.
2-In that same Help documentation, section 4.2 The Feff tab gives a link to the documentation of FEFF9. This gives the impression that the documentation is not very consistent.
All this said, one can now only wish that a Demeter version allowing the use of feff8 is released some day.