You could look into the Fax Service API and try working that into a .NET script within the Workflow. MSDN should have sample VB code for sending a fax somewhere, so that might be a good place to start.

However, depending on what you have in the way of a fax server/manager, there might be easier ways to make this work. We have a fax manager set up to turn email attachments into faxes when they are sent to a specific address/domain, which should also work with a workflow-generated email.

If you have the ability configure an email-to-fax service with an existing fax server, that would probably be a lot easier to configure and maintain than scripts in a workflow.