Instead of the provided sources I would like to work on another. Is that possible?

Yes, that is possible in principle. The prerequisite is that it is a structured source (i.e., it contains a repeating sequence of similar information). In addition, the source must be available in digitized form in good quality and free of third party rights. If the source is not public, data protection aspects may have to be considered as well.

You can find a selection of historic city directoy books that can be processed with DES here: http://wiki-de.genealogy.net/Kategorie:Adressbuch/Bereit_f%C3%BCr_das_DES

Desired projects can be reported to the project supervisor at the mail address DES@genealogy.net.