If the lamps are just part of the WMO’s geometry, you’ll need to edit the WMO in a 3D program like Blender and then re-add the lamps as M2 doodads.
EDIT: If you’re looking to extensively modify Stormwind, you might have better luck with back porting Cata’s SW to WotLK. It’s broken up into smaller chunks and uses more individual pieces than Vanilla-Wrath SW did. It also uses real terrain for the ground, allowing you to make make use of things like ground effects if you like.