Adding volume to my nextcloud.

Hello everyone,
I'm a complete newbie in the linux community. Trying to learn as much as possible while experimenting.

I can't get my head wrapped around the following issue:

My nextcloud installation on Linode runs on a Nanode 1GB. I use it for my company, and would very much like to expand the storage.

As it stands now, I have 25GB which is almost completely full. So I added a volume called "Addition". Afterwards I followed the following instructions:

Create filesystem on volume

mkfs.ext4 "/dev/disk/by-id/scsi-0Linode_Volume_addition"

Create mountpoint for volume

mkdir "/mnt/addition"

Mount new volume

mount "/dev/disk/by-id/scsi-0Linode_Volume_addition" "/mnt/addition"

Automatically mount the volume every start-up

/dev/disk/by-id/scsi-0Linode_Volume_addition /mnt/addition ext4 defaults,noatime,nofail 0 2

[Picture 1 as reference]
(https://imgur.com/BcfMVvA)

I was able to do all of this and the volume showed up in the nextcloud installation.

[Picture 2 as reference]
(https://imgur.com/NPm2w5B)

[Picture 3 as reference]
(https://imgur.com/uB4Bdui)

My question is: How can I add the "Addition" volume to the main Volume "SDA"? As it stands now, my nextcloud is unable to use the storage, and I would very much like to expand my initial SDA volume, so that it has 50GB of storage.

Thanks in advance!