Use
IWTE to convert the models from .cas then make the 2 models into one, using milkshape's merge function wont mess up the textures as all the materials will be kept along with propper UV maps. Then convert back with IWTE again, You don't have to alter textures if you delete part of the model or rescale it. There are probably a few mods using new strat settlement models, not sure if any are using RTW ones.
There are no tutorials for anything like this, and the only tool is the IWTE, on the specials tab.