The underlying data isn't customizable. Right now I think the best option would be creating a details sheet where the user selects a month from a parameter and then the view would show the data for 6 months previous to the user's selection. The attached workbook has an example w/ directions in the "Details" worksheet.
You might also be interested in this idea: https://community.tableau.com/ideas/3636
I hope this helps!
lookup filter.twbx 1.2 MB
Thank you so much for helping me out. This is indeed helpful!