With Outlook before the 2010 version, it was possible to specify any path for the signatures.
With Outlook 2010, the only way is to use those stored into C:\Documents and Setting\UserName\Local Settings\Application Datas\Microsoft\Signature\

I'd like to point the signatures to a network share. Allowing us to modify the signatures into the share, instead of login on every computers each time we are asked to modify them (and this is quite often because the signatures contain logos about current events).

We currently use a script to copy the signatures from the share to the local disk when users login.