There is nothing you can do to increase the cargo RATING, that is, the sticker on the door. There is much you can do to increase the load handling ABILITY of the truck, that is, what it is physically capable of doing. Upgrading the wheels, tires, axles, springs, etc. will indeed increase the ability of the truck to carry additional weight.
In your case, I take it your trailer's GVWR is 9,000 pounds. I would assume around 1,500 to 1,800 pounds of pin weight. Add to that the weight of the hitch and whatever passengers and gear will be in the truck. I think you will quickly reach 2,500 pounds. I would be looking for a more capable truck, not sinking money into the one you have to modify it.