That is definitely the best and easiest solution. First go to Insert > Symbol and find the one you want to use. Select the character code from ASCII (decimal). So for the section sign, it's 167. Close that and go to Tools > AutoCorrect Options. For the section sign, my abbreviation is SS. So type SS in the "replace" box (or whatever you want to use). Then click in the "with" box. You need to use the number pad for this part so if you're using a laptop turn on number lock. Now hold Alt and press the number sequence 0167 with the number pad. (Or put in whichever number is appropriate for pi, delta, etc.) That should take care of it. If OneNote / Word replaces your text in a way you don't like you can always click on the little icon that appears and flashes right after the replacement.