From the 1.84 release notes:
NEW! - When using historical players, there is now an option to import the players' real life stats achieved up to the initial season where PureSim is starting in history. This is useful if you want to start off with some prior history. These stats are treated just as if they were achieved in PureSim-simulated games, so they will be taken into account in career records, Hall of Fame decisions, etc...
So, for example, I started an association in 1960, and simmed one season, here is Koufax's player card (note how imported stats are colored as gray to make their origin clear.)
