I actually put that into one of my most recent maps, and here's how it works:
First, you have the player use the crowbar on the door.
Next, you have, in the map, a NON-ACTIVE "crowbar_joint" entity. Upon using the crowbar on the door, set the crowbar_joint active. Now, basically the joint works as a lever. The next thing you need to do is create an area near the edge of the joint's movement limit, such that, when the crowbar reaches the area, it calls a new function.
So, to do this, you will need the following:
AddUseItemCallback for your crowbar and your door;
AddEntityCollideCallback for your crowbar_joint and your area;
Inactive crowbar_joint in the map, preferably placed such that it looks like it is going in the edge of the door;
An area on the map placed such that the joint will enter it towards the end of its movement range;
A function to open the door and give it an impulse, which will give the appearance of the crowbar prying the door open;
Sounds and effects if you so choose (like Kyle said, take a look at the map for the guest room from the main game for more details on this stuff)
That should be about it. Hope that helped