First of all, you will need to add a commandbutton, like this ( you should change the image of the button anyway):
CommandButton Command_ConstructBarrowWightLair
Command = DOZER_CONSTRUCT
Object = BarrowWightLair
TextLabel = CONTROLBAR:ConstructBarrowWightLair
ButtonImage = BPCWightLair ; - here put your own image
ButtonBorderType = BUILD
DescriptLabel = CONTROLBAR:ToolTipConstructBarrowWightLair
Radial = Yes
End
Next, you will need to add this to the Mordor porter commandset:
CommandSet MordorPorterCommandSet
1 = Command_Stop
2 = Command_PorterExtinguishFire
3 = Command_PorterConstructMordorSlaughterHouse
4 = Command_PorterConstructMordorLumberMill
5 = Command_PorterConstructMordorOrcPit
6 = Command_PorterConstructMordorHaradrimPalace
7 = Command_PorterConstructMordorTrollCage
8 = Command_PorterConstructMordorSiegeWorks
9 = Command_PorterConstructMumakilPen
10 = Command_ConstructMordorTavern
11 = Command_PorterconstructMordorBattleTower
12 = Command_PorterConstructMordorFortress
13 = Command_ConstructBarrowWightLair ; this your commandbutton you've just made above
End
This should do the trick. You should also add these states for building anims, if you want it to be more like other buildings: AnimationState = BUILD_PLACEMENT_CURSOR ; AnimationState = PHANTOM_STRUCTURE
Hope it helps, also you may need to change the BarrowWightLair Side from Neutral to Mordor. Or you could just make a childobject for this one and make the changes there.