Hi Petr,
we had the same problem. But it was only one field mapping (cost center in line) where we needed a target value that has not been mapped (company code in header). For this single field mapping instead of using a nice AIF value mapping, we created a AIF value mapping function module, where we call the later value mapping (company code) at first and then the planned value mapping (cost center) for this single field.
A nice easy AIF solution looks different, but this was the quickest way.
Best wishes
Christoph