Two ways to get this.

The easy way is to just parent the imported props together. Choose one of the props to be the "Parent". Open the Hierarchy editor. Select all the "Children" then drag them to the "Parent". Now they will follow the movement of the parent object as if they were one.

To take this to the next level export the grouped object as a OBJ file. On the export option uncheck all the options except Include Existing Groups and Preserve Material names. Once exported re-import the OBJ. Again uncheck all import options, Now it's really one piece.