It's generally frowned upon, and almost always illegal, to re-locate turtles or release them once they've been in captivity. Part of the reason is that it's an easy way to transfer disease into wild populations. Usually only state-certified rehabbers and DNR or F&W are permitted to do such activities. I'd recommend that if you really care about ''wild populations'', you'll find one such wildlife rehabber. If you let us know what state you're in, we can assist with that. There are plenty of captive-bred box turtles for sale, so in the future please do the responsible thing and leave the wild ones in the wild, and purchase a captive-bred specimen.
Josh H