If you go into the world files, you can.
com.StefMorojna.SpaceFlightSimulator > Files > Saving > Worlds > select an existing world that is not important > WorldSettings.txt :
then you'll have something like this:
{"solarSystem":{"name":""},"mode":{"mode":0},"playtime":{"lastPlayedTime_Ticks":00000"totalPlayTime_Seconds":0.0},"cheats":{"infiniteFuel":false,"noAtmosphericDrag":false,"unbreakableParts":false,"noGravity":false,"noHeatDamage":false,"noBurnMarks":false,"infiniteBuildArea":false,"partClipping":false}}
and change the bold number:
0 = Classic Mode
1 = Career Mode
2 = Sandbox
Either of the Career or sandbox modes should have career parts.