No, you cannot
Presently, the real-time tests are time controlled and offered at scheduled date and time. Understanding the response from the users, we can customize to users convenience.
On the other hand, the mock tests are also similar in nature to that of real-time test and that can be taken at your leisure.
Please share your thoughts on your requirements, and we put in sincere efforts to make it happen. You can give us your inputs by writing to us at firstname.lastname@example.org