Up grades to a NT 5x8

Looking to build o 5 x 10 Belroy with off road tires on a NT trailer. Is there anyway to extend the length from 8' to 10' and add an axle capable of highway speeds wearing 15x32 off road tires? What about axle relocation? At what point would I want to relocate the axle and how do I do it? Thanks for any input.