Add the symbol to a dimension and then save it as a Style. I'm at home now so can't post a screenshot, but it's near the top of the property manager. I have a number of them saved in my drawing templates and it works well.
So this would probably be a kinda crappy way to do it, but if there's one of the existing symbols that you don't use much, you could edit your gtol.sym file and replace the unused symbol's definition with the Key definition. However, if you do that then anyone else who opens your files that doesn't have the edited gtol.sym will still show the original symbol.
I had the same problem and I solved it using Glenn Schroeder method.
Add the symbol to the dimension as you wish, then add it as a new style. It will be added to the drawing. Then save the drawing as template and you will find it on the 'Style ' drop down menu for future drawings.
Next time you need it:
and you'll get: