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I dont think that this can be done in current versions of Tableau, but there will be the option of wilcard unioning available from Tableau 10.1
I have just tested this using CSV files on Desktop 10.1 and when I add additional files to a folder, and refresh the extract, the new data is added.
I'm not sure how much this will help you, as all the files will need to remain in the folder so that all the data is re-added when the extract refresh is run, as opposed to appending new data (from the new file) to the existing extract...
Hope this helps