Hey piper,
first of all thank you! I had the project on hold so please excuse my late response. To harden the transform was the missing step i was looking for.
For anybody else who has a similar problem, here is how i did it:
Change one volume to be larger and so accommodate for second DICOM-Set
Volume Rendering – create new ROI->save
Converters->Crop volume->newROI as input ROI->save
3D Slicer: Volume Rendering and ROI's - YouTube, Merge dicom or nrrd or nii images, Padding a volume with CropVolume requires "Interpolated Cropping"
Landmark registration fixed volume is new enlarged volume, moving volume is the other DICOM
Use as few landmarks as possible. The last row is a live preview of the transformation.
Harden transform with Transforms module
https://slicer.readthedocs.io/en/latest/user_guide/modules/transforms.html
Combine volumes of transformed and enlarged DICOMs with Add Scalar Volumes module
thanks again Piper