The Lair doesn't have a button to be built? Then find a building that can be built, copy it, rename it, and replace the Object with the Warg lair Object name.
So would this work:
Command = DOZER_CONSTRUCT
Object = NeutralWargLair
TextLabel = CONTROLBAR:ConstructNeutralWargLair
ButtonImage = BIWargSentry_Warg
ButtonBorderType = BUILD
DescriptLabel = CONTROLBAR:ToolTipMordorTrollCage
Radial = Yes
End