The only semi-convenient way I've done this is by starting a new project with the same namespaces.....generate the "new" ones...then copy the .cs files over to my existing project, and copy lines of code for the mappings.
I don't think there is a "append-update" feature.
I would do the entire database (in the fake project)...and just have it "handy" for later....if you decided to bring in more items.
Sorry, its a poor-mans method, but just letting you know.