So essentially, you'd like to add another dimension member to Join_Ret? This would have to be done in the data source. One option, however, would be to find the calculation and place it on a separate worksheet, then place it on a dashboard next to the original sheet.
The separate worksheet is a good idea, but I need the percentage to be shown almost like a subtotal within each of the different values of the dimension. For anyone that's interested, I actually found this article online of how to manipulate the output of the grand/subtotals. Unfortunately, in my case, I have to recreate each measure and to show the percentage, I have to set decimals for some whole numbers, but it does create the effect I wanted.