It would be trival to tap the antenna on a USB key IF you can get the key itself open. A lot of small USB devices have their cases fused shut, which means if you want to alter them generally you have to saw or break the package open, you need a light touch to avoid hurting the PCB or a components inside, and then you have a naked exposed board to work with. Modifying the extender would be easier. Keep in mind, at ranges further than typical wifi setups (a few thousand feet) for you won't get a signal no matter what antenna you use if you don't also modify the settings in the registry for the software, they have a packet send/receive timeout that needs to be modified even if the RF front end can pick it up. I've heard of modified wi-fi setups in the country hitting 20+ miles line of site.