Yes, I can see the value in this idea too.
(I'm guessing that you're double-checking whether you've missed an existing way to do this before proposing it in the Feature suggestion thread, but I'll happily add a +1 there if you do propose it.)
To the DxO Developers, hows-about the following idea:
We already know that in almost all editing conditions we can use the SPACE bar to override the current editing tool to slide our image up, down, left or right or even to Zoom our entire picture, so I wonder if the DxO developers could arrange that - when the Crop tool is active - if we pressed the SPACE bar while the cursor was inside the Crop boundary, then the Zoom function would change the scale only of the four Crop lines, and thereby provide the behaviour the OP (Ralf) has described?! Conversely , if (in this imaginary hypothesis here) we pressed the SPACE bar while the cursor was outside the Crop boundary, then the Zoom function would behave as 'normal', to zoom the entire image while maintaining the ratio of the still-active Crop boundary?
Just a thought. There's probably a better way of achiveing this, but it's still a good idea I think.