@MelodyVeldorale @JasonWeis_Westwood sorry I did find the solution but forgot to update. The answer is as @JamesBurton1 pointed out, the "ArcGIS Coordinate System Data".
This program is available for download at My Esri, I do believe it is in use both for Pro and D2M and for sure a separate download together with D2M.
With the program installed I was successful when defining Image Coordinate System (ICS) heights as ellipsoidal and my Project Coordinate Systems (PCS) vertical as "ortometric" (NN2000 in my case). Was pretty satisfied to see the transformation in the processing report for sure 😄
In the installation directory you can have a look for what is available of geoids, in my case (Norway) C:\Program Files (x86)\ArcGIS\CoordinateSystemsData\pedata\vertical\europe\norway\geoid lists the 2 geoids I need.
Please reach out if you guys need more assistance, hope you find this answer helpful. Best pH