Forward spam to another email address

I have a dedicated email account just for spam (thanks, Otherinbox!), but most of the time I get e-mails in my spam folder from places where I never signed up, so I can't go anywhere to unsubscribe -- I would love to be able to create a filter that forwards all my spam mail to the other e-mail address, but none of the filter options give me the choice to forward spam. Any ideas?