The devs said it should work in the patch notes but haven't responded to ticket or forum...
v208.1
- Single Player Beta Mode
Note: Whilst not a formal system, Singleplayer users can play on any world json they want, by just replacing these files: \ShooterGame\ServerGrid.json \ShooterGame\ServerGrid\ (the map images)
When doing so, please back up your SavedAtlasLocal folder, and then delete your save data using the in-game save delete button.