It is possible to do so using the workflow customization however it will be complex. Check the TestSetFactory to do so.
As an alternative you should think of making the TS_USER_01 mandatory throght Test Plan entity and make it non mandatory using workflow for desired condition. This will overcome the limitation of importing data from excel and not marking the fields required (if they are made required using workflow).
As an alternative you should think of making the TS_USER_01 mandatory throght Test Plan entity and make it non mandatory using workflow for desired condition. This will overcome the limitation of importing data from excel and not marking the fields required (if they are made required using workflow).