in my understanding this is not possible. On the other hand, why not fully lock the employee? How long would such an update take? 1 min? Nevertheless, locking/unlocking is possible with following function modules:
HR_EMPLOYEE_ENQUEUE
HR_EMPLOYEE_DEQUEUE
Working via user-exit is similar as full locking the employee as the coding is called when using PA30. With PA30 the employee you've selected is locked.