I currently have a single seat of Inventor 2010.
Can I add a second seat to this version, if so, how and what would be the cost.
You would have to ask your Reseller. I believe the 2010 product line is expiring under Subscription with the 2014 release. Because of the industry and the non-compatibility between Inventor releases, it is one of the products that is best left on Subscription and upgraded yearly to stay current.