Copy & paste should work. I might be assuming Ctrl-V for the paste part.
The game just saves the key into a simple text file. It probably has already saved the bad key. You could just change it to the correct key. For Drox this file is at ~/.config/DroxOperative/User/key.txt
Din's Curse is pretty much the same thing with DinsCurse in the path. The expansions will end up in the same directory as the main game but tje file name will be keyExp1.txt
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2