Summary

Took apart a cheap BT Headset, made a case for the components and screwed it to the sennheiser HD201. :)

Instructions

You´ll need:
Sennheiser HD 201 Headphones
http://www.amazon.de/Sennheiser-500155-HD-201-Kopfh%C3%B6rer-schwarz/dp/B0007XJSQC/

Cheap Bluetooth Headset
http://www.amazon.de/gp/product/B005J47E6U

Beside the 3D printer, a soldering iron, screwdrivers, a small drill, glue

Take apart the Sennheiser Headphones and wire it like in this tutorial. But don´t solder the wires to a plug. Solder them directly onto the circuit board of the bluetooth headset, which you also have to take apart.
http://www.instructables.com/id/How-to-modify-a-2-sided-wire-headphone-to-1-sided-/?ALLSTEPS

Screw the bottom part with 2 screws on the frame, then feed the wires through and start soldering everything together.
Put the top with the buttons on and screw everything back together. ;)

Sennheiser HD 201 Bluetooth Headset