In my current project the default joist size was too small for the span, they were oriented in the wrong direction (down the length of the building instead of across its width) and there is no function by which I could add a support beam for unsupported spans. Unless of cousre I'm just not seeing all of these things somewhere!
I haven't played with the framing yet as I am only concerned with the visual appearance of the model so what is 'inside the wall' does not worry me, but I get the impression from reading the User Guide that it is pretty well automatic. There is a Custom Beam under Interior Accessories / Decorative. Not intended for reinforcement purposes obviously, but it may do what you want as an additional beam. You can of course add other 3DS objects to make into extra beams also.
Allan