Why add a second axle to an original trailer?

I was thinking the same thing. That is a lot of work to do and I have never heard of anyone doing it either.

I have seen the arguments over single verses multiple axles for years and could take up several pages arguing which is better. What I have came to the conclusion about it is what makes a trailer tow well is how well it is balanced over the proper number of axles it takes to properly carry the load. In over 60 years of being associated with all types and size trailers from small utility trailers to semi trailers and every thing in between the absolute most important thing assuming the proper tires and suspension is already in place is having the rig properly balanced and aligned. I have watched many people complain about how their trailer tows but after a quick inspection they have loaded it with to much weight either forward or backward.
As for the topic of adding a second axle, if I'm not mistaken it has come up here before but never carried out. 18 feet is usually the change point between single and dual axles on travel trailers but not always.
A friend of mine always preached multiple axles, then complained about tire wear, and cost of replacement plus toll road fees for the extra axles. I personally recommend having the number of axles with the proper tires to carry the load you are pulling and I like a nice safety margin in case I decide to throw something extra in. Also the size of your tow vehicle makes a difference in how you pull multiple axle trailers.