If the reference to a 500D close up lens is actually referring to a supplemental magnification lens to fit on the front a of a kit lens, as are often seen in adverts on Amazon/Ebay, these are normally on a 58mm filter ring, to fit the Canon kit lenses. The 100-400 II has IIRC a 77mm filter size, so considerably larger than the 58mm of the usual cheap supplemental lenses.
To be honest one is likely to get far better macro results from using a set of extension tubes, preferably the ones with the electronic connections, so that you can still control the aperture, and maybe get working AF too. As extension tubes don't actually have any glass, they do not have a negative affect on the image quality either, so cheap ones, as long as the electronics work, as as good as expensive ones. As they fit between the lens and body, they are not affected by the size of the front element either.
Alan