I've solved my problems. I created the package in two ways: extracting data from the deb package and compiling the sources. The first way doesn't work (with scangear, the scanner utility, it works: then there is something about CUPS communication). The second way works configuring all the source directories (and not only the ppd directory) with the option --program-suffix=mp250..
./autogen.sh --prefix=/usr --program-suffix=mp250 --enable-progpath=/usr/bin
where DIRECTORY is libs, cngpj, cnijfilter, pstocanonij, lgmon, backend, backendnet, cngpijmon/cnijnpr, pdd. It's important to make a link:
ln -s /usr/lib/cups/filter/pstocanonijmp250 /usr/lib/cups/filter/pstocanonij
if the CUPS printers page says that /usr/lib/cups/filter/pstocanonij doesn't exist. I've maked a package on AUR at http://aur.archlinux.org/packages.php?ID=33257
(see the build() function in PKGBUILD file for more informations if you don't use archlinux).