OK, I think I found a way:
Create an override in each group mask, move all nodes to the node network of the root override and start connecting.
This could turn messy with a lot of group masks but at least it works.

Is there another, simpler way that works directly i nthe shader tree?

Thanks.

Hello,

I am currently testing the Octane plugin for modo.

Quite often I set up the shader tree with a material at the very bottom and then overwrite the diffuse color with contants or image textures using group masks (item, part, selection set). This way I can get, for example, different colour ...