Why would you input additional additives? If the oil is at the end of its RUL, why not sweeten with additional T32 until you can do a full flush? How will you verify proper distribution of only additives?
Turbine Oils are base oil specific . The choice of Base oil is dependent on the oil's inherent demulsifying properties, and often called as TOBs. Or Turbine Oil Base Stock. Turbine Oils have a very small/tiny dosage of additives of the R&O Type, i.e. Rust inhibitor and Oxidation inhibitors. Due to nature of steam turbine application, varnish build up or contamination are two main cause of its deterioration.
Rather than introduce additives, you may well take up multiple rounds of filtration and reach a state of NAS 4 or similar cleanliness levels. Finally, a top up with fresh oil should largely suffice for meeting the existing operating conditions.