How can I add nest Master Pages?

Master page for your business's website.

However, inside that 'shell', you also want a certain design for each department of the business. All you need to do, is create the second 'Department' Master page. Then, inside the Master Directive, include a reference back to the Business Master:

<%@ Master Language="VB" MasterPageFile="~/Business.master" .....

Also, be sure to use different ContentPlaceholder ids, and remember the one caveat - if you're using Nested Masters inside VS.Net 2005, you'll need to only use the html view, since the design view does not support nesting master pages.