Hi Maru,
The "MIX"-option (if technically achievable) would fit nicely in our current workflow.
We always save our renders as CXR's. And do basic post in CIE, save conf files for each render, next we save out EXR's and do further post in Photoshop.
In the situation you have to add an object to an existing render and you don't have the time to re-render the whole image. It would be great to merge the "crop render" with the previous render in CIE.
The advantage of doing this in CIE:
-is that beside the beauty pass, the render passes will be merged as well with the new information. This cannot be done in Photoshop or other post software automatically AFAIK.
-and furthermore we can load the previous conf file on the merged render, save as EXR and replace the beauty render and render elements in Photoshop.