Question about Devolo Magic 2 LAN triple starter kit

You can, but every connection between the Magic 2 and the power line can reduce the signal quality. Certainly use a good quality, and short, extension cable. But the best solution is to plug a power strip into the outlet on the Magic 2, and then plug other devices into the power strip. The Magic 2's AC outlet is rated at 16A, the same as your wall socket.