Not a good idea if you want to live. After a while you will be breathing exhaled air which is not oxygen. I cannot remember the correct word/names for it. 

Paul 

Your lungs are not capable of pulling the air down past about 13" or so.

Have tried this and it didn't work, as have others.

It just doesn't work. A compressor is needed to build enough pressure to push it down to ya.

The deeper you go the more pressure you need.

Good Luck.

     OC

You can extend the draw pipe a couple of feet but if your inhaling and exhaling through the same pipe, youll have the usual 21% oxygen present on the inhale but only 2ish% oxygen on the exhale & the following breath.  You can get over this by exhaling through a vented mask but depending on conditions and the quality of your anti fog, you will most likely be fogging over in short order.  

If its just 2', I say just use the snorkel / breath hold / purge method.  Beyond that move to a surface supply or sling a bottle like a 30cf pony tank.

You could only do it by incorporating at least 2 one way valves. Kinda like a foot valve.One to let air in the snorkel and one for exhale. It could probably be done and cheaply but would require some engineering.