It is possible to open a keyboard layout that has been saved as part of a bundle, but not by double-clicking the bundle. You can either drag the bundle onto the Ukelele icon in the Finder or the dock, or you can launch Ukelele and choose “Open…” from the File menu to get an open file dialog to choose the bundle.

Either way, what happens next depends on whether how many keyboard layouts are in the bundle. If there is only one, then Ukelele will open that one. If there is more than one, you will get a dialog allowing you to choose which ones to open.