I did a similar modification on a single ended headphone cable. I re-terminated it with a 4.4mm balanced plug. I used a multimeter to figure out which cable is which and worked fine in the end. But if you're not handy with a soldering iron, it may be tricky to solder the cable ends to the plug, the connection parts are close together.
For my HD600, I bought a custom cable over AliExpress for about €25. It's 2 meters long and looks pretty good too. There are multiple options there, where you can select the plugs, cable types and length. The only downside is, the shipping takes really long.