When you create a signature in Outlook, it actually creates three - one for HTML, one for rich text, and one for plain text.
You can edit these files directly by navigating to %APPDATA%\Microsoft\Signatures. (Paste this into the address bar of Windows Explorer, or go to Start > Run, and paste there and click ok).
I don't recommend editing the HTML one unless you know what you are doing. Create the signature first in Outlook, then just leave the HTML one as is. But you can open the .txt ones with Notepad and the .rtf ones with Word and edit them.
This will enable you to have three different signatures for the three different email formats.