Welcome mate. Yes you can replace the Holden badge on the grille with an Opel badge. There are two ways to do this. One way is to get a whole new Opel grille, looks identical to the Holden grille but with an Opel badge. That's the way I would do it personally. The other way is to just remove the Holden badge and stick some black plastic behind the Opel badge so that the black Holden badge mount outline can't be seen. Some people choose to do it that way.
Enjoy.