[DOSBOX] Mapping keyboard keys to RetroPad

I wanna play some Dosbox games on my Wii U. One of those is Raptor (a shoot’em up) which uses just a few keyboard keys like CTRL, ALT, the arrow keys and so on. How can I map them so I don’t need a keyboard on the Wii U? For example: LEFT_CTRL=a LEFT_ALT=b LEFT_SHIFT=x SPACE=y ENTER=start

and so on. Of course this has to be done per game and I think instead of putting the exe files in the playlist I’d have to put some launcher files (cfg files?) in there which execute a specific exe with a specific mapping?

bumping No one? This would make DosBox fantastic on so many systems (especially on the Wii U)!