Yes. You can upload lineups you built somewhere else or by hand, and on Ultimate, Contest Sims grade them against the field for your contests the same way they grade lineups SaberSim built.
Right-click the build tab and choose Upload Pool.
Uploading lineups to a build replaces that build's Lineup Pool. Rebuilding lineups after uploading lineups will also overwrite the uploaded lineups. If you want to keep a pool you already built, start a new build for the imported lineups.