Sure! Just because you made the mistake of starting somewhere doesn't mean you should pay for it all year long!
Just create a league, edit the draft-board to reflect your current rosters, and select complete draft to get your league started. While we won't be able to copy over your prior wins/losses or accumulated points data, you can still play the rest of the season on Sleeperbot and have your commissioner add your prior record manually to calculate new playoff seeds.
It's a bit of a small workaround, but keeps you from having to stay on existing platforms for making the mistake of not drafting on Sleeperbot to begin with :).