Windows 7: Trying to Add digital signature in Word

Hi,

I am trying to add a public digital signature in word but am unable to do so. when I try to protect the document and go into 'protect document' --- 'add a digital signature' and selct change to change the signature it only displays my local signature (myemail@company.com) I would like to use a different signature like a public or trusted signature.

Is thier any way I can do this?

Your help will be greatly appreciated. thank You

Some information about Digital Signatures here:
Add or remove a digital signature in Office documents - Word - Office.com