It's not possible. There's nothing behind the door, probably because the game designers wanted to make the mansion seem big without having to put in pointless rooms.
Even if you were to hack it, there is nothing there. (The door to the very right with the maid in front, yes?)
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you cant, well you can if you have a walkthough cheat on
You can't get in there, but if you have Action Replay you can walk straight through the girl and get in.
On the west side of route 212, in front of Mr Backlots mansion there is a couple who own one!Although this is nothing to do with the question, notice that Backlots name can refer to two things:His BACK garden has LOTS of PokemonHe owns a gardening LOT outside the BACK of his house
You can't get the Secret Key in Pokemon Platinum without Action Replay, unfortunately. You can get Action Replay from most major websites.
No, you can't mix records in Pokémon Platinum in order to get the Secret Key from a friend.