As Andi has mentioned
These are the steps (please note this is from an 8100 but the menu selections should be similar)
I understand that you have a company issued Blackberry running off of a BES, not BIS.
When you are in your email folder select the menu > Options:
Now look for Email Settings:
Look for "Use Auto Signature":
You should be able to change anything that is there and add your own
Regards,