It's the default config which doesn't seem to have that feature. So . . . I could change the kick feature (which is not used) to admin_toggle_mute_participants. This is stored I confbridge_additional.conf, but if I replaced admin_menu by setting up the same in confbridge_custom.conf it should override it, no? I can't see somewhere to edit this in freepbx.
CLI> confbridge show menu admin_menu
Name: admin_menu
*=playback_and_continue(conf-adminmenu)
*1=toggle_mute
1=toggle_mute
*2=admin_toggle_conference_lock
2=admin_toggle_conference_lock
*3=admin_kick_last
3=admin_kick_last
*4=decrease_listening_volume
4=decrease_listening_volume
*6=increase_listening_volume
6=increase_listening_volume
*7=decrease_talking_volume
7=decrease_talking_volume
*8=no_op
8=no_op
*9=increase_talking_volume
9=increase_talking_volume