Hi MrM1,
The only answer to your question is both yes and no. What you are suggesting is called a dominating layer (firmer layers over softer layers) and it’s used to create a certain “feel” that some people prefer who like the “feel” of a firmer sleeping surface so in this way it will “feel” firmer. There is more about the effects of dominating layers in post #33 here and the posts it links to.
On the other hand if you are looking to create firmer support for your mattress because the heavier parts of your body are sinking in too far and you are sleeping out of alignment then there is no effective way to do this by adding layers on top because firmer top layers will still bend into the softer layers below them so any benefit would be partial or temporary at best. The only effective way to make a mattress that is too soft (in terms of support) any firmer and more supportive would be to replace the comfort layers that were too thick or soft with firmer or thinner layers and/or to replace the deeper support layers with a firmer material. There is more about “fixing” a mattress that is too soft in post #4 here.
Phoenix