Somebody already did that infamously a few years ago. Some guy on the Spawn forums claimed to have found Alternators 'Black Lucky Draw Sideswipe' in a Dead End box. It was pretty obvious that he opened it, stuck a Maisto Dodge Viper in the box and resealed it.

Some idiot ended up paying over $180US or so for it. When he got the toy, WHAT DO YOU KNOW? Its a Maisto die cast car and not a Transformer. When members of the Allspark asked the buyer, he claimed that he and the seller agree that Hasbro was trying to screw customers and its all Hasbro's fault.

Yes, they (claimed) to believe that Hasbro randomly puts toys made by rival toy manufacturers in their packaging to screw customers around.

Jesus.