Hi ryoung,
the Terra with the topper was one of our favorites as well when we were testing mattresses for ourselves.
There are many good sources for latex (see post #4 here) and if you ever did decide that you wanted to add a layer you could go in any direction you wanted. You could add a topper on top using any type of latex or ILD that you wanted, add a new firm layer underneath (although I would question why), exchange the layers in your mattress with something else, buy a thicker cover and put 4 layers inside your mattress, or any combination of the above. At this stage though it’s unlikely that it would be necessary and I would focus for now on making sure the layers you have work well and cross that bridge if you ever come to it 
Phoenix