Can you free roam after you beat dead rising 2

No and Yes.

YES:
You can free roam and kill zombies up until your 72 in-game hours are up.
Don't worry about ignoring Stacey or Not giving Katey Zombrex as these actions only affect your ending. They won't result in game over, all that happens by doing this is that you get ending D or F based on your actions.

NO:
If you want to get a good ending (EX: Ending S, A, B or C) you can't just free roam around the place you have to stock-up/ get Zombrex for Katey, play the case missions, etc. But, like I said you only have to do this if you want a good ending.