I want to add things like basic tables/chairs/houses I have made over the past year to my ShiftA menu. Is it possible to do this?
There is an addon called creaprim which allows you to do this.
Select the object which you wish to add to the Add menu and press Create Primitive in Tools shelf > Tool > History > CreaPrim:
You can now add it from the Add menu:
You could create a group Ctrl-G and set the object's name for each of your objects in Object Mode. To add groups to the scene use the Add Menu Shift-A Group Instance.
In case you would like to add your objects from python to the menu you could use the
add_curve_extra_objects addon as a template you find it in the Blender installation directory in
2.70/scripts/addons/add_curve_extra_objects (where 2.70 is the version) see
To include your own objects in the add object menu you can create an addon. There are a few example addons available that you can look at, as well as a simple example script available in the python templates of blender's text editor.
While these examples use python code to create new objects you could have your script import an object from another blend file.