*facepalm* Aha, sorry. It's still morning (well, it's not, but for me it is) and I'm only half-way through my first cup of coffee. Am a little dense.
You have to write it, since you don't have the editor...
[font=georgia]YOUR TEXT GOES HERE[/font]
You can substitute Georgia with any of the offered fonts. What I tend to do is try this in a PM, where I have the editor at hand, and then just copy+paste into the sig area.