We have a exiting login page for our web application. Right now it doesn't support windows authentication so I guess I need to add a domain drop down to the page.  Can I utilitize the existing code in the login class to populate the domain drop down or do I have to write my own code for this?

I don't think there is a method exposed to populate that drop-down list.  But, it is easy to do with the classes within the System.DirectoryServices namespace.  There are Active Directory objects that allow you to get the current domain, and recursively get the parent of that domain until you get to the top of the forest.  Then, just go through the child domains until you have a complete list populated.