Actually, there is a way to add new textures to a composite texture you already created. Let’s say you created composite texture A for building A and you wanted to add textures from building B to texture A. Do this:
1) Open db A, bring building B into A (copy/paste, x-ref etc)
2) Select the faces of building A (that presumably are all mapped with texture A).
3) “Add to Select” the faces from building B.
4) Start Texture Compositor
At this point, Texture Compositor will include texture A and all “composite-able” textures from building B. Now simply position textures from building B in the new composite texture. Note that if texture A had some “blank spots” Texture Compositor (by default) won’t let you put any textures from B into those blank spots. Hold down ALT key while dragging positioning B textures to fill these empty spots in texture A if desired.