Hello,
someone in Germany seems to be able to modify a SA-21 to perform scans of
more than 6 images (just like an SA-30). At the extreme price of an SA-30
this would be interesting to be able to do.

Does anyone know how to do this?

You can check out the offer (in German) at EBAY:
EBAY item no: 5153483794
http://cgi.ebay.de/ws/eBayISAPI.dll?...e=STRK:MEWA:IT

Roland