I don't understand the question. Can you go into more detail and perhaps provide a use case?
I guess you want Custom SQL Parameters, which is a new feature in Tableau 8.
I wonder if this thread has a workaround Parameterize and/or build Custom SQL statement in a Data Connection?
Ps. Online help for the feature in Tableau 8: http://onlinehelp.tableausoftware.com/current/pro/online/en-us/help.htm#customsql.html