Need to do a bulk enrollment for the existing employees

We need to migrate existing employees and enrol to benefit plans, please suggest us ways to proceed with this bulk enrollment, as when we used participant enrollment it was showing us the error stating "enter started life event dates" but if the employees are new to the benefit plans, how there would be an existing life event as the employees would be new to the enrollment.
Tried loading the benefitsenrollmentupload but faced error.
Attaching the excel sheet for reference.