One way is to delete the corner except for the vertices located nearest the green line that create the profile.
Then, place the 3D cursor where a line through the vertices at each end of the corner intersect near the red X, set the pivot point to the 3D cursor, select and extrude the vertices near the green line and rotate the extruded vertices a short distance around the 3D cursor, and repeat the process as needed enough times to get the look of the corner you want.
I've uploaded an annotated screen shot
from a blend file in which the default cube is scaled down along the z axis, and offset along the x axis (1) to represent the top of the door, and the offset cube is duplicated (Shift Dkey) around the z axis to represent the the side of the door (2). The end of the top element is duplicated (using SHIFT Dkey), and moved a short distance to form one end of the corner (3), and the process repeated with the side to form the other end of the corner (4). The x co-ordinate of the newly created end of the top of the corner, and the y co-ordinate of the newly created end of the side of the corner, are used to create a vertex at the center of the arcs of the corner which is used to locate 3D cursor. Locate the 3D cursor at this vertex, and set it to e the pivot point (5). select all of the vertices of one end of the corner (3), extrude them, and rotate the extruded vertices with rotation constrained to the z axis of the pivot point (6) several times to create a smooth radius on the inner and outer sides of the corner piece. Create faces between the last extrusion and the other end of the corner (7).
As an alternate to setting the 3D cursor at the center point of the two arcs forming the corner of the door, one could select the center point itself, and use that as the picvot point by setting the "active element" as the pivot point. However, sometimes extrusion deselects the active blender element, necessitating reselecting it before the next extrusion.
I know it's outside the scope of your question, but you only need to make this mesh once. Once you've made it, you can duplicate it, and move and rotate it to place it at the other three corners, or you can use a modifier (mirror or array) to achieve the same thing.