Thanks for your anwers!
Creating an extended crosstab as Mustafa mentioned is for sure the cleaner and much better solution.
Nevertheless there are some (also commercial) extensions available for DS which do actually exactly that what I'm trying to do.
But, of course, Michael's objections are absolutely valid, thank you for making this clear.
Is it possible to extend the standard crosstab of the design studio?
Creating a custom one (or extend for example SAPUI5's table control) is nothing that can be done in a few days, I guess...