A consideration; How long until this additional 20T of storage is also consumed ?
Why not schedule the down time and convert to LVM (Logical Volume Management) ? Then is no problem at all in the future to add additional storage -on the fly-.
Else; one is looking at formatting, mounting, deciding on what "data" the new storage is to hold, copying, reconfiguring to point to the new partitions, testing, then removing the original data on the original storage. Only you can know what "data" moved would be the least intrusive to your users. Only you can know what reconfiguration will be needed to do this.
we can help in generalities..the specifics are on you
simply said, but is a huge task