Team admins can add a fixture directly with an opponent, date, kick-off time and ground. If you don't set a ground, it defaults to your team's usual home ground (or your club's, if the team hasn't set its own); you can override it for an individual fixture any time, useful for one-off away days or neutral venues.
Fixtures can be edited right up until they're played, and results and stats stay attached once you've recorded them. See Running matchday for the full walkthrough from squad selection through to final stats.