You need to modify virtusertable to add the alias and either access or relay-domains to specify the domains. These files (which could be named slightly different) are most likely in /etc/mail/ . This site ( http://www.defcon1.org/html/Software...usertable.html
) explains pretty well the steps. While a BSD site, sendmail is sendmail, as long as the features are compiled in (usually), there would be no problem. More help is available if you google for virtusertable.