When using the “Insert” command to import a 3DM using the settings “Link” and “Reference”, the greyed out “> (layer name)” that results cannot be a sublayer. You can move it to become a sub layer as you work, but opening the file the next day will move it back to the top layer level at the bottom of your layer tree. I suppose there is no fix to this as of yet, is there?
The whole functionality of having referenced 3DMs is great though. But why do they have to be blocks?