You can make this possible by using variables.
Make two hotspots of the door. One closed image, one opened image. You need to make one integer variable (ex. door). Then made the first one to be shown if the var. door is 0 and the opened if it's 1.
Open the first hotspot (closed). Put the var. door in the second thing: Show this hotspot only if... put the var. door in the 0. So, if it's 0 it will be shown, and if it's 1 it will be hidden. Open the second hotspot (opened) and in the Variables tab in the second section down, put the var. door in the second row, where it's 1. So, this will be shown if it's 1.
Then, drag and drop the item in the editor to the first hotspot (closed) and go into Variables tab. Put the var. door to be changed to 1. So, when you use the key to the closed door hotspot it will change to 1 and the door will be opened. Made the same with the other hotspot - drag and drop the key to the second hotspot and put to change the variable to 0...
Edit: I just say that you need just to open the door and I explained the whole process of opening and closing. It's the same, just in half
Tell if you have some problems and sorry if I didn't explain well...