Reaching through a doorway to hit the ball
On a court without outside play, you lose the point if any part of your body is outside the court when you hit the ball, and that includes a foot in an open doorway. Where outside play is allowed, it is fine.
The rule
Unless outside play has been allowed, you must be inside your own court when you hit the ball. If one foot, both feet or any other part of your body is outside it at the moment of contact, your pair loses the point.
Where the doorway ends
At an opening without a door, the edge of the court is the bar fixed across the floor, which counts as part of the frame. If there is nothing on the floor, it is the line straight down from the frame above, painted or not. A foot past that line is outside.
Where outside play is on
On courts built and approved for it, you may hit the ball with your feet or body outside the court, and the return is good if it meets the usual requirements.
質問
Can you stand in the doorway to hit the ball in padel?
Only where outside play is allowed. Otherwise, any part of you outside the court at the moment of contact loses the point.