There is NO problem with adding Super P to the mix at the job site. On a general basis, Super P is only active in the mix for approx. 45 mins AFTER dosing. After that you will have a rapid set due to the w/c ratio. Job sites add Super P at the site to assist in prolonging the workability time...
Just 1 more thing to remember BigH. One difference that is often overlooked is the amount of moisture each method allows. That is a Modified Proctor will not allow as much moisture as using a Standard Proctor on the same material. Also a 95% Modified is close to being the same as 98% Standard.