Uses the Unix server the same e-mail domain like your Exchange server?
If NOT, this is an easy task:
Create a new SMTP connector with the unix mail domain as address space (like *unixmail.local). Enable routing for this connector and configure the unix server as smarthost.
If you want to address the unix users from your GAL and/or from the internet you have to add contacts for them. Create a contact with the exchange address as secondary, and the unix address as primary SMTP address. Exchange will forward all mails for this contact to it’s primary address.
If the the e-mail domains are equal you have to do something more. The “Exchange Server Interoperability and Migration Guide” shows you some hints and solutions. At least you will have to configure a different domain for your unix server.