Simply have Prussia aquire more Polish land earlier on, if for example early Brandenburg-Prussia somehow gained Greater Poland/Wielkopolska, maybe even a bit more, then a significant part of their population would be polish and that would be hard to ignore. But Poland was more powerful than Early Brandenburg-Prussia, so that's hard, gotta jump together with Sweden/Russia.
Historically Polish was accepted, the problem as Prussia gained more land in Germany was that Polish became a more and more insignificant minority. Imagine if say they gained the land of roughly the modern borders of Poland before gaining the Rhineland, it would be close to a 50/50 German/Polish-Schlesian split, and the state would either crash and burn or be forced to adapt to both cultures.
Another options, and probably better for a long term strategy would be if somehow the Poles could be made to think of themselves, like the Czech did, as both Poles AND Germans. Ofc the rise of nationalism kinda screws with it, it had the tendency to ruin everything.
Have 20% of their tax base come from Polish provinces.
Haha. Prussia just need to pick Humanist ideas and it will be trivial to get Polish as an accepted culture, especially with how much base-tax Poland has now.