Testing further, it seems that each Geometry Nodes tree creates its own Action (and first slot when needed). After that, you can add any slots you want.
Without Geometry Nodes, it’s possible to create an empty Action and just add whatever (Object/Material/Shader) slots you want.
Since I very rarely use more than one Geometry Nodes tree in a project, I guess it isn’t a practical limitation. Just important to do things in the right sequence.
