To do it: Make an event with the door closed (or use your door currently), click on the blank space to make an action, on the bottom of the first page of actions, on flow control, click the first action, conditional branch. After that, click on "4" tab, then click on the circle on the side of "items". Get the key item from the items, then click it. (assuming you created the item) then click ok. Then click on the blank space below "Conditional Branch: (item or key name) in Inventory. After that, click on show text in message tab, if you want to add a text if he want to open the door, and write anything you want. Now, click on blank space below and click on "Show choices" below "Show text", leave it like that, and press ok. Below [yes], click on blank space, then click on control self switch on game progression tab, and then click ok. Go down where you can see "Else", and below that, click on blank space, go to tab "3", then click on Game Over, in Scene Control tab. Now, all the way up in the window, click on "New Event Page", then go down on "Conditions" dialog and click on circle aside of "Self Switch", then leave it like that. Now, if you want to add a opened sprite, go down and click to get a picture. Finally to pass the door, click and enable "Through", just below the event picture. What you make in event page 2 is interaction with door after the door is opened.
A simple conditional branch will do

it's like this: