You can launch the Installation Manager and modify your RPT install options to add PVU support. The following link describes that setup – it looks like your particular situation applies under option 1.
As far as how many virtual users you can take advantage of, that depends on your CPU. The PVU licensing only applies the number of virtual users that your processor can support and still give you valid results. However, you will have to check your license agreement or with your IBM rep to see if you can apply your license as PVU. See this link for more info about licensing, including PVU.