Pierrot,
You can't in PPT. But you can work around it using AutoCorrect.
First insert the symbol in a textbox using Insert - Symbol. Now close the
symbol dialog box. Select your inserted symbol, go to Tools - AutoCorrect
options
AutoCorrect tab. You should see your symbol should be in the replace with
box. Type a set of characters for instance yy, something you are not going
to use in a normal text and click on the add button.
Now everytime you need the symbol type your set of characters hit the
spacebar and you will get your symbol.
Luc