That is a bit complicated because Outlook saves the signature file to the local hard disk and this signature is specific to the Outlook Profile that is in use. In this case it will be a bit hard to share it within the GAL. However, if you are looking at applying a standard message to all messages, it is easier done through the IMSext.dll which is confgiured through the Registry of the Exchange Server. Let me know if you need more info on this.
Kuria