DestroyFunc FvwmSetAllwaysOnTop
AddToFunc FvwmSetAllwaysOnTop
+ "I"  Layer 0 6

DestroyFunc FvwmSetNotAllwaysOnTop
AddToFunc FvwmSetNotAllwaysOnTop
+ "I"  Layer 0 4

And in your window parametre menu :

+   "Set on top"                    FvwmSetAllwaysOnTop
+   "No longer on top"                  FvwmSetNotAllwaysOnTop