GVWR

Not necessarily, as the structure of the vehicle may be at its gross weight rating limit, with the existing axles already present.
But in a "general sense", yes, the more axles on a vehicle, the greater the GVWR "should" be.

Hard to answer that question with a definite yes or no, as more details need to be known, or in what context it's being applied.

 

Usually whatever company does the work puts a upfitter tag on with the modification made, in that case it would also include a new GVWR for the vehicle.

 

^^^ It's one of those CYA things if you want to add extra axles to your rig. They will change that little tag in your driver's side door jam that was put there by the manufacturer, showing the new GVW rating.

 

There's a little more to it than that - you have to factor in bridge law, as well. Bridge law typically benefits longer wheelbases, which is why you don't see old COE's pulling the 28' framed end dumps. Also, bridge law is going to be the law of the land - the Ford F750 has a manufacturer's GVWR of 38k.. cross a scale at that weight and see how you fare.