How to write manipulated image back to negative?

Hi all,

I would like to do some simple dng raw image manipulation like black image subtraction.

Therfore I though I would get the stage2 image (because working on stage1 is not  correct right?) of both the main-image and the black-image and create an image "main minus black".

Now I want to have this image saved back as dng as if it was the main dng.

just setting the new image as stage2 does not work because writeDNG uses the stage1 image which is set as fRawImage in buildStage2Image.

Setting as stage1 does not work too because it is already a linearized image.

How can I reverse the linearization?

Or are there other way to do this correctly?

Thank you all and best regards

Andreas