Theres NO WAY it can be done. The magwell dimentions wont match, the recivers on most high end Automatic Electric Guns " A.E.G " or Gas Blowback Rifles " GBR's " are made with cheap metals. Some of the points from one of Ohio's airsoft shops are as follows
1) The original seizure was not because the lowers were in violation of NFA rules, but for their auto sears.
2) There are going to be similarities between real guns and airsoft, because they're designed to look like real guns. (common sense)
3) Those similarities don't mean that if you buy and airsoft gun, you're going to be able to use it on a real steel gun. (more common sense with some trial and error thrown in)
4) I can guarantee that if you put a WE lower onto a real upper, it will not fire a bullet. (personal knowledge of the differences between the WE lowers and my real steel lower)
5) Said upper MAY... MAY be able to be modified to shoot a real bullet for limited use before failure, but not without extreme effort to do so. (you can probably make any object fire a bullet if you try hard enough and have the equipment and knowledge to do so)
6) One guy comes out and says it IS possible, and suddenly it's the gospel of the media world. (why? Because it's a sensational story.)
7) There is a difference between fact and fiction.
heres the link to the same topic on airsoft ohio
http://www.airsoftohio.com/forums/showthread.php?t=28644