Thanks to Mohammed Hassan now I know an alternative (alternative to the official howto) way to generate a .deb package for Maemo.

If the package already exist in the Debian repositories, you can get the .dsc file (for example in an ftp like this: http://ftp.debian.org/debian/pool/non-free/s/spim/ ) and execute the following commands:

dget -x DSC_FILE_URL

It will download the package and will unpack it in the current folder. You have to enter in the created folder and edit the debian/* files to personalize settings, mantainer data, add deps ecc...

When you're done, you can generate the package with the usual command:

dpkg-buildpackage -rfakeroot