We would recommend using dietpi-drive_manager to attach, mount external drives. Although, /etc/fstab is also fine.
If you want the Samba Server to point to the USB drive, and, you used dietpi-drive_manager to setup the USB drive, use dietpi-drive_manager or dietpi-software to move your DietPi user data, from SD to USB drive. Samba will then point to USB drive. https://dietpi.com/forum/t/dietpi-software-user-data/400/1
Or edit /etc/samba/smb.conf and change path = /mnt/dietpi_userdata accordingly.
Not within DietPi currently, you’d need to edit this manually in /etc/fstab.
If you do the manual edit, you will also need to replace the symlink for /mnt/dietpi_userdata and point it to the new mount location, else, Samba server location will point to nowhere (rootfs).