I would like to know this aswell.

There is:

But i dont know if that is actually what i want. Is there a way to send joystick-movements as dpad actions to a core (that does not use the joystick itself)? If not, im willing to take a shot at it, but i am not really sure where to start. Thanks!

Edit:

Forcing mergeDPADAndLeftStickEvents==true will allow the left pad to act as a joypad. Is there a way to expose this setting?

Edit2:

                var allowJoypad = controllerConfigsState.value[port]?.mergeDPADAndLeftStickEvents ?: false
                when(system.id) {
                    SystemID.NES -> allowJoypad = true
                    SystemID.SNES -> allowJoypad = true
                    SystemID.GENESIS -> allowJoypad = true
                    SystemID.GB -> allowJoypad = true
                    SystemID.GBC -> allowJoypad = true
                    SystemID.GBA -> allowJoypad = true
                    SystemID.NDS -> allowJoypad = true
                }

                if (allowJoypad) {

Inserting that snippet in the file linked above will make it work. The list above is probably not complete, but it should be safe for them to get merged inputs.