I don't think you can use HeaderFooter when altering an existing document - you need to work with lower-level classes. I also think that PdfStamper is a better choice than PdfCopy.
Here's an example that adds text to an existing document: http://1t3xt.info/examples/browse/?page=example&id=24 (that would be in the "getOverContent" part). Instead of setTextMatrix and showText you may want to use "over.showTextAligned(PdfContentByte.ALIGN_RIGHT, text, x, y, 0)" with suitable values for x and y (which will depend on the page size).
They weren't very bright, but they were very, very big. Ad contrast:
a bit of art, as a gift, the permaculture playing cards