i think they do stack to create a function. there are 4 main functions that are accessible by an individual and there are 4 that are not. take an ISFP for example. your functions are Fi, Se, Ni, Te. that leaves you with 4 other functions unaccessible. i believe, yet can't prove, that you can create pseudo functions out of your accessible functions, like the example you gave, but really the last 4 functions really aren't worth looking into. i also believe its not worth wasting your time on; it wont tell you all that much. i push this a lot but using just the temperaments will get you quite a bit farther then the functions will. the functions explain how the temperaments are derived, in a way, so why not just stick with the temperaments? also if you are going to look at the functions you need to look at them like a team and not as individual entities. as individual entities they really dont do to much. they also tend to be influenced by the other functions around them. Ni in an ISFP and Ni in an INFJ are going to be quite a bit different.
sorry for the random line of thought. i just typed what i thought, hope it makes sense.