There is a door in LMB guarded by a couple neutral (yellow dot) T'rang.
If I step too close they become hostile. What's behind that door?
Can I wax them without the entire T'rang Empire becoming hostile?

Helix

before the place where they tell you to go away, go to combat mod, run towards the door, open it and run inside. you can see what's inside then.

however, there's nothing interesting inside.

There's an experimental mind blast rod in it. A polearm that only psionice, monks and bishops can use, and that casts nuclear blast al level ~3.

It is possible, but not easy, to get it without aggravating the T'Rang, but IMO it isn't worth the effort.

You'd have to find some monsters nearby, and lure them to this room. The guards might 'retreat' and leave this spot, guarding their new area instead.
Then you can open the door. Enter combat mode, run in to grab the rod, and run out immediately. If you manage to leave the room (there are more T'Rang gurarding inside), combat ends.