How can I add items to the right-hand panel of the edit dialog?

I’m not sure what terminology to use here, but look at the screenshot of the dialog window that appears when I double-click a tag such as XMP:Subject. The right-hand side of this dialog looks like it could be populated with a list of items, I’m assuming for quick access. Is that possible? Or is this portion of the dialog not really used for anything? I’d love to be able to put frequently-used items there, if possible.