Welcome to the forums. I did a simple search of these forums and found this answer for you at http://community.vzw.com/t5/BlackBerry-Devices/Signature-on-text-messages/m-p/175778
Hope this helps,
As we say down here in Mississippi, "There's more than one way to skin a cat," there's more than one way to add a signature to a Blackberry device. You can add an automatic signature to all of your emails. Do the following:
1. On your device, go to Email Settings.
2. Select the email account you want to change.
3. Select Edit.
4. Under General Options, scroll down to Signature.
5. Change the default "Sent from my..." to what ever you want. You can place your name and address in the field.
6. Save..you will be prompted for your password for your email account, so make sure you have it.
7. Every time you send an email, the Signature field will be placed automatically at the end.
Remember, you won't see the signature, but it will appear on the recipient's email. If you have more than one email address associated with your device, you will have to change each email account, but that is where copy and pasting comes in handy.