These trays are very similar to those of the WarpCenter. As opposed to the WarpCenter though, there are no limits as to how many trays can be defined per tray widget, and you may add more than one tray widget to each XCenter.
Note that not all widgets can appear in a tray. Most notably, sizeable widgets cannot appear in a tray. In the "Add widget" menu, those widgets that are not trayable are automatically grayed out.
As with the main XCenter, in order to create an object button widget, you will need to drag and drop a WPS object onto the tray.