Can I build knowledge models with PoolParty which have more expressivity than SKOS provides?

Yes, you can extend the SKOS schema if you like.

You can introduce new properties like ‘works for’, ‘lives in’ or ‘is part of’.

You can also add any kind of metadata property on top of a concept and you can even enrich each concept by additional properties automatically from Linked data sources, e.g. geographical longitude and latitude on top of concepts which represent places.