This is something we have been requesting for a while so no doubt it will come some day. Door knobs should be easy as they are available on cupboard doors, but the complication obviously is the fact that the door can be opened, so the knob has to move too.
In the interim you may like to try the attached knobs that I made in TFP. One is a single knob the other is two which should be fine on most thicknesses of door and saves adding two to every door. The image is of the twin knobs, with the door bezel set to 50% mirror, so it is reflecting the timber floor etc. If you want to use this, just import it into your catalogue and you can use it like any other element. To import it, right click in the section you want it (eg Interior Accessories) and select 'Add Element' and select the "Import Geometry from File" option. Browse to the 3DS file and during import set its size approximately. You can change it exactly when it is in your catalogue and set the colours. When setting the Appearance Mat0 and Mat2 are the knobs while the others are the bezels and barrel through the door.
You can also import elements by going to File / Import / Object Wizard but don't forget to set the 'type of element' as this is where it will save if you want to add it to your permanent catalogue. To add it to the catalogue (not just the current model) go to File Catalogues / Save Element to Catalogue and click on it.
Needless to say you will have to positon the knobs on the door at what ever angle you have the door (turn off Collision) and Elevate to the height you want (or set that in the catalogue). They obviously will not move if you change the door angle.
You can also search the web for '3DS door Knobs' (or SKP - Google SketchUp format). You will find quite a few. Just bear in mind when you are choosing one, that some are be very large files, for use in the top professional 3D programs. Some are so big that they can be a larger file size than your whole house, so they have the potential to slow things down somewhat.
I hope that helps (until such time as door knobs come with the door!)