ok, in short : look into /usr/share/doc/libsane, there is a file README.debian which gives instructions how to use the proprietary plugins.
libsane-extras contains an epkowa backend, so you need to extract the plugin from the rpm and copy to /usr/lib/iscan. However, the backend is outdated, if you have a scanner, that requires a newer version of epkowa you are doomed because the above mentioned dependency.
So I filed a bugreport
against this dependency and, as usual, debtard developer tells me to complain to upstream without further explanation and closes the bug.
So I can only guess what the problem is. Probably the proprietary drivers challenge his religious believes about free software.
Fortunately my scanner can be used with the included backend, so the thing is done for me, no reason to fight windmills. At least I've tried ...
If you have a scanner that requires a newer epkowa backend, go file a new bug but get your shitshield ready. Talking to a debian developer about proprietary software is like talking to a taliban about anal sex ...