Using papers published before admission to partially fulfill PhD requirements
Is it realistic to expect that these papers can be used to partially fulfill PhD requirements?


Probably not, if you're in the U.S.  There's a concept of a "Ph.D. by research", which is awarded on the basis of past research accomplishments.  This only barely exists in the U.S., although it's more common in some other countries.

Instead, the way most U.S. Ph.D. programs work is that the dissertation must be based on research conducted as part of the program.  One reason I've heard is that overseeing research in person makes it easier to judge how well or honestly you are carrying it out or how much assistance you might be getting.  In any case, being unable to use your previous papers in your dissertation is generally not a real obstacle to graduating quickly, if that's your goal.  Much of the time in a Ph.D. program is spent developing the ability to do high-level research, and if you already possess that ability upon entering you could finish substantially more quickly than usual.  (I'm not convinced that finishing quickly would be a good career move: it's almost always much better to focus on depth rather than speed.  However, you shouldn't worry that putting your prior work off-limits for your dissertation will be a burden.)