It is not so simple..it depends from which version you starts
17C88 or 17C89 or 17C205 or some beta needs to modify platformsupport.plist
You have to add there ImacPro1.1 board id
Instead if you download a full Imacpro1.1 High Sierra version you do not need of any adding in platform plist because new imacpro 1.1 board is there by default
Fabio what alterations did you do in uefi bios (if you did) and in the folder EFI (patches, etc) to run this SMbios? In my case, if I try to change from mp6,1 to imac pro the system gives KP and does not run...