How would/could one go about taking a door and the appropriate jamb and putting them together, so that they become a single mesh, rather than two individual ones?

Download NifSkope, and load the door file and jamb file. Then right click on one of them and select Copy Branch. Then in the other, right click and select paste branch. This should make both files be in the same window. Now you must right click and select edit and move them to fit each other. Then save the file. For more information you can read http://www.gamesas.com/index.php?/topic/955345-wipznifskope-tutorial/