Yes, a switch statement is an efficient way to do this. Basically, for each sf::Keyboard key, you have a corresponding df::Keyboard key.
Remember, the reason for doing this is to avoid having the game code, which will access the df::Keyboard value, know anything about or be tied to the sf::Keyboard value. As for the time it takes to right this, you can make an "empty" case, cut once, and paste it in for each key. Then go through and fill in the keycodes. Doesn't take too long. ;-)