The Thingworx Model and it's built in storage structures are not able to be extended to another RDBMS at this time. For version 6.0 the model will always be neo4j and data can be held in DSE Cassandra or neo4j.
You can however use the Thingworx jdbc connector (using a driver provided by the RDBMS vendor) to connection to any outside data store. You can then build in structures to store and/or query data (and make other SQL calls) to that system. This is something that needs to be setup/built/configured for each outside data store on a case by case basis at this time.