Unfortunately no, it cannot.
The front I/O panel of the Define R6 is a totally new design with a different shape from any previous Fractal Design case, and the Connect D1 is specially designed to fit that unique shape.
The Connect D1 also makes use of existing cables in the Define R6 for power/reset switches, LEDs, audio jacks, and USB 3.0/2.0 ports. While this makes the Connect D1 more material-efficient and easier to install in the R6, it also means it is incompatible with other cases due to differences in the shape, layout, and orientation of the buttons, ports and jacks.
Three different color shells are included along with everything you need to install the Connect D1 in any version of the Define R6, but it is physically incompatible with other Fractal Design cases.