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When a data source is published to Tableau Server and used as a conduit for issuing queries through Tableau Server to the database, it is known as a Data Server data source. The underlying data connection may have multiple tables, but the data source itself presents a single, flat table structure under the internal name 'sqlproxy'. Since Tableau Server proxies the entire data source and not any of the specific elements of the underlying connection, it is not possible for users to manipulate the underlying connection in the fashion you have described. Instead they must download the data source from Tableau Server and modify the connection within Tableau Desktop. This is easy to do from Desktop -- simply click the download icon instead of the link for a given data source in the Tableau Server connection list.
I encourage you to post a new Idea in our Ideas forum and explain the functionality you desire and how it would be used: http://community.tableau.com/community/ideas
I hope this helps,