You may read here about subd fuse feature: https://discourse.mcneel.com/t/subd-union-help/100628/10
It was released in v7 in GH but it`s still WIP project.
In Modo if you want do booleans on subd then you should freeze subd to polygons mesh (high poly mesh) and do your booleans then. In Modo there is also Mesh Fusion which works on subd with adding some fillet but its also freeze it to polygons automatically. I think that Blender do it in same manner. You should freeze subd to ordinary polygonal mesh before booleans operation.
For me your question is quite strange because If I have to do subd booleans then I convert subd to nurbs to have nice blends. When you do booleans on freezed subd mesh then you have only sharp corners without nice blends between shapes. Thats why I think its good to convert subd to nurbs when you would like to have blend between you shapes. I think that even Mesh Fusion in subd polygons won`t give you so nice blend with almost unlimited width.
I’ve made that subd blend below with converting it subd to nurbs and creating pipe in intersection curve which sliced both sources and I’ve made blendsrf after that to get smooth bridge between. I think it`s best way of merging subd with nice blend.
