How do I assign Control_R+Menu to the global kxkb switch action?

"Remap the Menu Key (like this post) :

Put keycode 135 = F14 (or any other F-key or whatever) to ~/.xmodmap. This make the computer thinks that the Menu Key is the F14 key.

Then you put a script in the Autostart-folder (usually ~/.kde/Autostart) with the following content :

#!/bin/sh
xbindkeys &

Make the script executable (chmod +x filename).

Either logout and in again, or just run the script.

OR

Like this post, try running the following command (which has the same effect, but I don't remember if this is permanent) :

xmodmap -e ""keycode 135 = F14""

If 135 is not the right keycode, referring again the second link I provided :

Try running the command xev in a Konsole, and press the Menu key.
Look in the output of xev for keycode n, where n is the keycode of the Menu key."