Use HTML to set your fonts. IF you don't know html, you could put your text into an Excel range and then use Ron de Bruin's RangeToHTML().
https://www.rondebruin.nl/win/s1/outlook/bmail2.htm

He has a routine to use the signature html file as well.
https://www.rondebruin.nl/win/s1/outlook/signature.htm

There are 2 other ways to insert the signature.