I have been involved in the airsoft and paintball industries, and it is a blast.
I have talked to some of the guys that are "really into it" and they do say that yes some of the weapons could be converted to a real gun, (though I do not know of any in existence) but you would be replacing so much parts/pieces that it would be more work than it is worth. The GBB (Gas Blowback) style rifles are about the only ones I can think of even being able to possibly do it. Most are metal construction, (even some of the AEGs [Automatic Electric Gun] are metal contruction, but I still dont think they could handle the shock/trauma that a real round delivers. They would still require more work than they are worth. Might as well go buy a real one, or build your shovel gun.
Either way I think that seizing guns for that reason is pretty damn stupid and ridiculous. They look very real, that is the point, but to seize them for that reason, and that they "could" be converted is pretty stupid.
My friend brought up a good point, the ATF agent states that "if in the wrong hand they can be converted to a real weapon" well in the wrong hands any weapon, or anything could be used for the wrong reasons......