Yes you may
You are allowed to do your PICUS training from the left seat, and the captain does not have to be a line training captain. You can log all the flight time as picus, who has the controls at t/o or landing does not matter.
But, it has to be agreed and approved before flight commences that you are flying as PICUS.
This "method of supervision" is acceptable to my country's authority (JAA-land).
Remember that you are only allowed to fly as many PICUS hours as you require to obtain 250 hours PIC/PICUS for your ATPL.
Have fun!