Nope. No need to change anything at all. Just use DrivePool to create a pool using your existing CloudDrive drive, expand your CloudDrive using the CloudDrive UI, format the new volume with Windows Disk Management, and add the new volume to the pool. You'll want to MOVE (not copy) all of the data that exists on your CloudDrive to the hidden directory that DrivePool creates ON THE SAME DRIVE, and that will make the content immediately available within the pool. You will also want to disable most if not all of DrivePool's balancers because a) they don't matter, and b) you don't want DrivePool wasting bandwidth downloading and moving data around between the drives.
So let's say you have an existing CloudDrive volume at E:.
First you'll use DrivePool to create a new pool, D:, and add E:
Then you'll use the CloudDrive UI to expand the CloudDrive by 55TB. This will create 55TB of unmounted free space.
Then you'll use Disk Management to create a new 55TB volume, F:, from the free space on your CloudDrive.
Then you go back to DrivePool, add F: to your D: pool. The pool now contains both E: and F:
Now you'll want to navigate to E:, find the hidden directory that DrivePool has created for the pool (ex: PoolPart.4a5d6340-XXXX-XXXX-XXXX-cf8aa3944dd6), and move ALL of the existing data on E: to that directory. This will place all of your existing data in the pool.
Then just navigate to D: and all of your content will be there, as well as plenty of room for more.
You can now point Plex and any other application at D: just like E: and it will work as normal. You could also replace the drive letter for the pool with whatever you used to use for your CloudDrive drive to make things easier.
NOTE: Once your CloudDrive volumes are pooled, they do NOT need drive letters. You're free to remove them to clean things up, and you don't need to create volume labels for any future volumes you format either.
My drive layout looks like this: