I think they can - I would hope Jennings for example has been working for his for example since he was dropped. I think what has made Hales, Roy, Buttler the players they are is working on their technique every minute of every day for ODI cricket. Hitting the ball out of the ground, hitting it 360 degrees and all of the qualities associated with ODI batting. It's possible for really good players to learn the skills to play in all 3 formats Root, Kohli, Smith, Stokes and others) but there's a danger that a) They just don't have those qualities b) Working on them thoroughly might take something away from what makes then fantastic ODI players.
Just my opinion.