It's like a scenery, but you need to separate the moving parts and make an animation for it.
First you model the door as separate objects where you want differently moving parts. One frame is the parent that the animated frames will link to. You also link objects that don't move to it. Then you have other individual frames, where the moving parts are individually linked to.
Left half of the door linked to 'frame left'
Right half of the door linked to 'frame right'
Threshold of the door that doesn't move (if you want) linked to 'frame door'
frame right and frame left are linked to 'frame door'

Also export a collision model, making sure to make separate objects for each part and linking them in the same way.

Then you make an animation, where the start of the timeline is closed and the end is open. I think. Or the other way around.

Compile, look at other door tags, etc.

Do you know basic animation, like, just the timeline? Press N to start animating, move to the end of the timeline, and move objects to where they would be when they are open.
Just like this: http://www.youtube.com/watch?v=DdKG3o4MVqI
I think you save it as a .jmo

Now what about for those of us that are unhappy with the door's model, but are pleased with its animations? I modified the b40_giantdoor to fix some of the shading and shape irregularities, but ran into a mismatch of the animation graph and the model. I tried re-exporting the animations, but I still get a mismatch. A search on halomaps and google under multiple inquires and strings has resulted in little help (though I did learn the differences between animation types).

Note: this is my first go at animations of any kind, so I suspect I could be running into a simple error caused by a stupid mistake. However, my research thus far has not verified that suspicion.

Use kornmann's guerilla and make the model checksum the same as the animation checksum.

Amazing. Worked like a charm.