Hi Ken,
If I understand, you want to homebrew your campaign setting and create custom classes without hacking the core rules, right? If so, then sure, lots of GMs do that. Torchbearer is very flexible.
However, the systems and the economies are tightly coupled and interwoven. If you haven’t played much to begin with though, I would try to master the core game before hacking it, but I know a lot of people do it the other way around, and then learn from their mistakes after something breaks. Either way can work.