It is easier to buy a trailer that is made for the load you want to haul.
Unless you are a manufacturer, I would not even think about messing with changing axles, welding more metal, etc.... because if you are ever inspected (more and more random trailer inspections around the country to get the unsafe trailers off the road) and found out that you have modified it without having it re-registered after having DOT inspecting it, it can easily be impounded by the DOT officers. It only gets worse if you are in an accident.
When you look at the math, an axle can be $130.00, brackets and hardware to install it $35.00, new hubs $50.00 and now brakes $130.00 for that axle and then misc. welding services to have this done if you are not a certified welder, maybe $120.00, then tires and wheels if you don't match up the same bolt pattern, probably $200.00. When you look at it this way, it might be cheaper to buy a larger trailer, transfer your equipment to the new trailer then sell your old trailer.
I thought about modifying my old trailer a few times but after doing the math, it is easier to buy the correct trailer for what I would need then sell the old trailer.