Enabling “extra” keyboard layouts from base.extras.xml & evdev.extras.xml under XFCE

In addition to the keyboard layouts which by default can be selected in the xfce4-keyboard-settings app, there are many listed in /usr/share/X11/xkb/rules/base.extras.xml and /usr/share/X11/xkb/rules/exdev.extras.xml, which are not available for me to select. How can I enable them under XFCE?

(Related: Enabling SIL phonetic Biblical Hebrew keyboard, which addressed this on GNOME-based systems. The answer given there does not work under XFCE.)

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