<p>Anyone know how to do this without using a third party program?  If there no way to do it with a add-on someone can recommend one?</p>

<p>EDIT: I need to add this in the server so all users have the same signature.</p>

<p>Thanks</p>

You need to create your own exchange message sink to do this. Here's a classic VB example from MS KB:
http://support.microsoft.com/kb/317327
and a VB Script one:
http://support.microsoft.com/kb/317680
And lots of goodness from MSDN about Exchange 2003 Transport Event Sinks:
http://msdn.microsoft.com/en-us/library/ms526223(EXCHG.10).aspx
If you're running Exchange 2007 then you can use Transport Rules:
http://msexchangeteam.com/archive/2006/12/12/431879.aspx
http://www.msexchange.org/tutorials/Using-Transport-Rules-Creating-Disclaimers-Exchange-Server-2007.html