I simply mounted a drive (for me it is iscsi, for you it will be your SSD) to the bitcoin volume mount.
sudo mount /dev/sdb /home/userfolder/umbrel/app-data/bitcoin/data/bitcoin
After that add an entry into fstab and reboot to test.
~$ sudo nano /etc/fstab
/dev/disk/by-id/ssdid /home/userolder/umbrel/app-data/bitcoin/data/bitcoin ext4 errors=remount-ro 0 2
~$ sudo reboot
Then log back in and run df -h command which should list all the partitions and sizes
~$ df -h
Filesystem Size Used Avail Use% Mounted on
tmpfs 393M 2.2M 391M 1% /run
/dev/sda1 16G 5.7G 11G 36% /
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
/dev/sda15 105M 5.3M 100M 5% /boot/efi
/dev/sdb 984G 446G 488G 48% /home/userfolder/umbrel/app-data/bitcoin/data/bitcoin
tmpfs 393M 4.0K 393M 1% /run/user/1000