Default [Q] external keyboard: set custom modifier key

Hey everyone,

I'm searching for a external keyboard for my android-stick. My big problem is those which fits my needs are in QWERTY-layout, but I'm from Germany and therefore would like to be able to type umlauts (, , , ).
Because many of the keyboards designed for Android don't have ctrl or alt keys, my idea was to define a custom modifier key, like the space bar for example, so that when I press space+a it would result in .

Is this even possibe with Android?