The c++ solution provided by voodoo_jannick is only applicable in version 5.4+, as the ShouldVirtualAcceptSimulateMouseButton boolean does not exist prior to 5.4.
I found a workaround using Blueprint. On the OnKeySelected function of the InputKeySelector, you can pass the Selected Key to a function which checks what the input type is and return a key based on this. If input type is Mouse/Keyboard then return Left Mouse Button, or if it’s Gamepad then return Face Button Bottom.
I can’t add more screenshots because Epic thinks my account is new, but essentially you can just plug the output of the OnKeySelected event to this function. Then you can update the InputKeySelector Mapping using this key
This works for my use case. Hope it helps someone out there.