It's possible but I'm fairly sure you'd need to sacrifice training to make that work, because with only 5 players you need a lot of Stamina and GS training. If you desperately want to promote you could try it, but personally I'd stick with your plan. Even with the best preparation and players, promoting is never a guarantee, you can always get unlucky. Just keep trying and incrementally improve your team. Make sure you have high enthousiasm and game shape when you enter the playoffs, and homecourt advantage helps a lot too.