Page Number Format on Existing PDF

I just have a quick question, I have been looking and haven't found a way to accomplish this so far.  We have some existing PDF files that we would like to add page numbers to.  I have found how to do this, but the problem is I cannot find how to change the format of the page number, we'd like to use different formats (such as I, II, III, or A, B, C) instead of just the typical 1, 2, 3.  (Using a PageNumberStamp)