Yes, if your training permits this sort of intense format and it is in the best interest of the client, assuming the client wishes to do so.
Please be aware that if a therapist is found to be pressuring clients into more sessions or offering unnecessary sessions solely for monetary gain, their account will be suspended.
Once you have agreed a date/time for the additional session with your client, you can book this on the system for them.
There does not need to be a reserved slot in place for you to book a session for your clients, as you can book for any dates/times you have agreed with them. Whilst you cannot add more than one reserved slot for a client, you can edit your working times to remove any times that you have reserved for them each week to prevent any new clients from being able to book that time.