Unless someone in the community has more direct experience with the product, I am going to provide some rather general information. Some packages of Invensys PRO/II provide OPC Server capabilities. In that case you would be able to use PI OPC Interface to bridge the gap in the context of what OPC offer.
In another direction, there are different add-on packages to basic PRO/II for integration purposes. So you might want to know what is available in what you have. On the PI System side, on top of PI OPC Interface there is this whole Data Access suite at your disposal. It includes PI System SDK, PI OLEDB and PI OLEDB Enterprise for SQL type data access as well as PI Web Services for web queries.
Can you be more specific about the PRO/II modules that you are working with. Is it for refinery units and models?
For a quick start, you can use PI DataLink to bring suitable values to Excel and then export to load the current or historical values into the simulator.