I'm not quite sure why we can't use Xinput. This is a Unity engine game, and it supports it.
There's a plethera of information on how to do it, however as far as you'all have come with this game I'm certain you don't need help.
^ Not what's needed to add it, but just proof that it can be done fairly easily.
Please implement this. Most controllers now a-days are Xinput. Only few have the option to toggle from D-X, and vice versa.