Couple ways to achieve this

1. You can disable the auto email population feature. By default, alerts  are restricted to the email address stored in the user  profile and subscribe all your users  using a email alias or distribution list.

a. For sharepoint  Portal Server go to Site Settings, Manage alert settings, and uncheck the box “Always use user profile field ”. Then go to the content you want to setup alerts and subscribe an email alias or distribution list

b. For Windows Sharepoint Services, login with a user without email address, select the content to be alerted, enter the email address of the alias or distribution list


2. Programmatically, you need to build either a web part or some code that will use the object model to subscribe users