[koolkat]

Assuming I have very good crafting skills, 
 
Can I pick apart a pair of closed Senns cans, punch some holes in the outer shell of the cups to make it "open-air" headphones? 
 

 
like this
 

 

 

[blufox4900]

I don't know why anyone would want to try since Sennheiser already offers a wide variety of openback headphones. That being said, I suppose you could.

 

[BotByte]

But that already has holes.
 
Really, closed drivers are usually different then open for a reason. Yes, you can drill holes, but it won't add much or anything at all by doing all that work.
 
Why would you want to anyways? I make them open or just bored?

 

[koolkat]

Just bored.
 
So the internals of closed headphones are a lot different from open headphones,
not just a piece of plastic covering the cups...
 
It'd be cool if you could mod Sennheisers. 
 

 

[Mad Max]

You may or may not ruin the sound, it will depend from model to model.  You will have to play around with the dampening in the cups after punching the holes.
I would just get an open-back headphone if you want to try one.

 

[Uncle Erik]

Unless you've done a lot of acoustic design and understand how it works, you'll probably just make them sound worse.
 
Why not try something like the DIY electrostats?