Boot from SD or external hardware device?
You always have to modify boot.img, so the answer is no. If you considering to edit boot.img and manage to mount the external disk to /system, you should mount another partition to /data too.
There are apps like Multirom out there which can loop mount ext4 image files located on internal storage. Afaik not for pie, you should try with some old device first before soft bricking your S9+