Shane

Yes, DrivePool allows you to use pools as disks for other pools.
For example if you had physical disks A, B, C, D you could create a pool E from disks A and B, a pool F from disk C, and then a pool G from disk D and pools E and F. Note however that any given disk or pool can only be directly part of one bigger pool at a time (so you can't "add" disk A directly to both F and G).
Keep in mind that exactly like physical disks added to a pool, pools used as disks don't share their own contents with the new pool, only their free space (with the new pool being a hidden folder in the file system of the contributing pool/disk). So any content you place in the example pool G would not see any content you placed (outside of its hidden folder) into disk D or pools E or F.
For a more specific and relevant example, say you had drives A, B and C on one computer and drives D, E and F on a second computer. You could create a pool G of the first computer's disks, a pool H of the second computer's disks, and a cloud I formed from G and H. Then you'd be able to move your files into the cloud I while independently controlling duplication on each of the two computers by adjusting G or H appropriately.

Hi Querl28. There's a few different ways.
Simplest is you install the replacement drive, tell DrivePool to add it to the pool and then tell DrivePool to remove the old one from the pool. DP tell you whether it successfully moved all the files on the old drive across (in which case you can then physically remove the old drive) or not (in which case you have to decide what to do about it).
If you don't have spare ports to add the new drive before removing the old one, but you have enough free space on your other drives in the pool, then you can tell DP to remove the old drive from the pool before you install the new one.
See also this support page on removing drives.