A 4k (4096) cluster size supports a maximum volume size of 16TB. Thus, adding an additional 10TB to your existing 10TB with that cluster size exceeds the maximum limit for the file system, so that resize simply won't be possible. Volume size limits are as follows:
Maximum Partition Size
This is unfortunately not possible, because of how Cloud Drive works. However, a simple option available to you is to simply partition your drive into multiple volumes (of a maximum 16TB apiece) and recombine them using DrivePool into one large aggregate volume of whatever size you require (CloudDrive's actual and technical maximum is 1PB per drive).