All you need to do is create a dummy (or whatever floats your boat) at 0,0,0, link your object to it, then export them both together.
Though I have to ask, it looks like have one object that you are animating with subobject keyframes? that is probably not the best (though probably technically possible via morph targets) for in-game use. Idealy, you’d want to have the model rigged properly so the slats don’t deform badly when the doors open/close. At the very least, I would have each side as its own object, and then scale them (as objects, not subobject) as needed - though this will result in the same bad looking deformations for the slats. but at least that way i imagine it would be a lot less expensive than doing it as one big morph target.