From what I gather Mr Crabs you can't write a new mapping file from afreash in Cross
What you will have to do is to cheat a little. At the bottom of the controller mapping list is a option to "Dubicate" a controller mapping.
Choose a controller and Click on the dubicate option, and Cross should generate another mapping for that controller which you are able to edit and change
Once you have this Dubicate map, all you have to do is open the mapping list and choose a option for example "Load Deck" then hit the button on your controller or that action.
I hope you get what I mean
You can also import a controller mapping into Cross but I haven't looked into that yet