Combine Wall with Sloped Stair Railing Wall

[Dominick]

I created stairs going to a 2nd floor loft (story and a half home), and created a sloped railing from the walls on either side of the stairs. There is a very small "landing" at the top of the stairs, which is part of the floor of the room on the 2nd floor. I want to put a closet under the stairs, but I cannot combine the first floor wall with the sloped wall railing to put a door there (I can only put the door in the wall, or in the sloped wall railing, but cannot insert a door that is located around the intersection of these two walls).

Pics are attached. The 3D pic shows what I'm after in terms of the stairs and "landing", and the 2D pic shows the break in the walls between the main 1st floor wall and the sloped railing wall to the left of the stairs. I want to hinge the door in the main wall, but allow it to overlap to the sloped railing wall.

TIA,

Dominick