On my PC, signature file is saved at \WINNT\Profiles\Administrator\Application Data\Microsoft\Signatures
(I am the only user on my PC and I login as administrator.)
You need to specify this file in Options|MailFormat, Signature (at the bottom)
If you are the administrator of the network, you can copy this file to a specific path on all workstations and then let your user know to specify the file in Outlook.
I have tried searching in registry... found here:
HKEY_USERS\S-1-5-21-1790861235-857282123-569397357-500\Software\Microsoft\Office\10.0\Common\MailSettings
There were 2 string values, "NewSignature" and "ReplySignature". Both had the value TPS (name of my signature).