I am not quite sure why you are quoting the stack documentation when you say you want to install a new add-on.
If you have to apply a newer support package stack before installing the add-on, then, yes, I would let SUM do the SPAM/SAINT update as well.
If you don't need to apply a newer SPS, then you can import the SPAM/SAINT update without SUM.
Either way you can install a new add-on via SAINT and should make sure to have the latest SPAM/SAINT update installed.