There should be absolutely no problem adding a primary filter. The flow rate to the engine is only a few GPH. The fuel pump will not notice the additional filter. Even with only one filter, the vacuum pressure is very low until it gets dirty. Once your on engine filter is dirty the vacuum builds. Your fuel pump is designed handle that load perfectly well. As a note, all marine diesels are delivered with only the on engine filter. All builders put a primary filter ahead of the secondary on engine filter. If you instal a Racor 500 or 900 filter you can get a recording vacuum gauge to tell you when the filter is getting dirty. That will alert you to change the filters before they get too bad and before the vacuum builds. Given its age, your engine probably was delivered with a 10 micron filter. A 20 micron primary ahead of your on engine secondary would be a good set up. When the vacuum builds in the primary you know to change both filters. My 2009 cumins has a 2 micron secondary. I have a 10 micron primary with recording gauge. I change the secondary filter once a year (we run about 600 hours per year) . I change the primary every 2-3 years (1,200 to 1,800 hours) , even though the vacuue gauge is still well below any level of concern. But if I did get a fuel problem, the recording gauge would let me know right away.
RK 23284 Stainless T-handle vacuum gauge Kit for all Turbine Series