You can create you own 3D objects with a 3D software (sketchup, Autocad, ...), and use them in 3DView, with some rules to respect:
1. origin
Create the objet around the (x=0; y=0; z=0) point
2. scale
In 3DView, 1 unit makes 1 meter (around 3,3 feet), but this might be different in your 3D software.
So the easiest is to make some tests: you save your OBJ file with different sizes, you watch the result in 3DView, and you chose the best size one.
3. size
Unlike 3D object creation software, 3DView has to refresh the gobos in real time, 10 or 20 times per second. Accordingly 3DView can handle dozens Kb objects, but not hundreds Kb object. This means it will not work if you try to import a very nice object created by a grafic artist.
If you respect the above rules, you can copy/paste the OBJ file in one of the folders in "../YourSoftware/3DView/other/". And you can import it from the "Objects setting" menu.