Citrus doesn't generally ferment very well, nor vary tasty. It's actually pretty darn nasty!
I do use frozen fruit juice concentrate in my apfelwein recipe. I either add additional apple to a batch, or will add cranberry or cherry to a finished existing batch both to sweeten and provide some adjunct flavor. This is either in conjunction with wine conditioner (which stops the fermentation), or on its own - which means that the added sugars will eventually ferment out, leaving the apfelwein just as tart as before.