Make sure that the field in question is setup as a Rich Text Field. Now when the user is filling out the field they can hit the Ctrl-E combination and the Field Text Properties toolbar will popup. This will allow the user to change fonts or add styles to the text in the textfield. They can change it back after the fact if they wish.
I find it easier to put all of the text in (using one font). Then put the curosr where you want the GDT symbol to appear, hit Ctrl-E and change the font, then add the symbol you want. You don't have to do it that way that is just the way I would do it
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Hope that helps
Paul