Mathias, as you have noticed there are two basic ways for ThingWorx to integrate with Windchill:
1. Using WindchillSwaggerConnector extension Windchill Patch for WindchillSwaggerConnector
2. Using Windchill Extension from Marketplace ThingWorx IoT Marketplace
Both include Windchill as well as ThingWorx side extensions that includes REST services in Windchill that allow for querying certain Windchill data and displaying that in ThingWorx mashups. The REST services I believe are not extendable at this time so there is no supported way to add new one.