All Stilo racing helmets come with a military specification locking socket for the earplug connection to the Stilo helmets electronic wiring system.
This conversion socket can be used with all ST5 Race helmets for the connection of non-Stilo earplugs with a 3.5mm jack to the Stilo helmet.
Alternatively, you may want to look at purchasing the Stilo Ear Plug kit which comes complete with the relevant male connector to allow easy Stilo connection.