Hello,
You state you currently have a P2000 controller enclosure and a additional drive enclosure. You are planning to add another enclosure. To address whether you can add the second drive enclosure hot or not mostly depends on how you wish to cable the array.
With performance (straight-thru) cabling, adding an enclosure while the array is running is fairly easy and straight forward. I would still recommend a period of low host IO as hte array will perform several rescans to reform the backend SAS map when the new enclosure is added.
If you are going to cable the array for redundancy you will need to perform the drive enclosure addition carefully if you are planning to do this hot. You would want to add the A path first, allow the rescan to complete and successfully see the new enclosure and drives. You would then want to recable the B path for reverse cabling as per the HPE MSA Cable Guide. Again this should be done during a period of low host IO.
After the addition of the new enclosure be sure to confirm the firmware version and upgrade to the latest.
Cheers,
Shawn