3ds max how to animate door for unity

hi,

i have model a house in 3ds max and i want to animate the door and windows . So i position pivots of the door and windows to their respected areas.i want a single bake texture map for everything.that's why i attached everything.now my problem is......that when i go to (Element) sub-object mode and select the door for animation(open and close),the gizmo or pivot change its position to center.I try link x form modifier but its not worked in unity. i know there is an easy way to do this; Please help me.... :(

thanks in advance : )