Is this question related to Alfresco or DFS in general? I.e. I would assume that if you mount a DFS directory in a Windows client it should be able to index it on the operating system layer.
But if you are asking if Alfresco can "index" a DFS directory then the answer is no. Alfresco only indexes content that is managed within the application. An external / arbitrary DFS directory cannot be indexed. What you might do is import the content stored in the DFS directory so the files / folders are accessible as content in Alfresco, and then they are also indexed via the Alfresco search functionalities. But immediately following this process, you should stop any direct / user access to the DFS directory and always go via the Alfresco repository / applications to work with the content.
