The us grilles are all one piece, though it might be possible to cut the Isuzu waffle out and set the Opel insert into it. If it was the only way for me to get one, I'd vote for taking the insert out and cutting the surround in half with a single vertical cut in the center. A small brace could be installed on the back side and when it gets painted to match it could be finished discretely.
I'm serious about the CIH diesel too. It would have to get here for less than or similar $ to the JDM Isuzu 3.1s that are @$2k.