The Shared Desktop Ontologies were written with a very broad purpose in mind. That being said, they are still cases that are often not covered by it. In that case there are two possible options for extending the ontologies

Contributing to SDO

Creating a custom ontology

Most of the ontologies are written in the trig syntax.

In order to install an ontology, the follow files are required -

ontology-name.trig The TriG file containing the ontology
ontology-name.ontology.in A file describing the ontology

In order to ease the process of creating custom ontologies a template ontology is provided in kdeexamples/nepomuk/ontology