This would be better served by a video but since I don't do videos I'll take a stab at explaining the process.
1) Draw the circular room with the curved exterior wall tool and place a straight exterior wall just outside the circle. (thumbnail #1)
2) Select the straight wall and move it just inside the curved wall so that there is enough space to accommodate the door width. (tn #2)
3) Place the door in the straight wall. (tn #3)
4) Use the break wall tool to put a break in the curved wall on either side of the door opening. (tn #4)
5) Delete the curved portion of the wall in the doorway. (tn #5).
6) Select the straight wall, click on the "Connect Walls" tool and then click on the curved wall. The walls will merge. (tn #6)
7) Finally you can select the straight wall and drag it out closer to the edge of the curved wall. (tn #7)
And that in a nutshell is how I did it. Of course this is just a proof of concept approach.
To recreate all this to exact existing dimensions will probably pose additional problems.
I wish you luck.