I don't think you can...
but it would be a nice wish for the inventor idea station !
As already mentioned you can not presently edit to add symbols.
If however, you can find a font with the symbol you need you can use the Character Map key to include it in your text.
As mentioned this would make a good feature enhancement request at the Inventor Ideastation.
Hi,
Here you have the existing link for this idea.
Thanks for votes!