Can Fusion drive SSD be paired with external HDD

My late 2013 27” iMac’s Fusion drive has lost its HDD side; it spins morbidly slow. I suspect it choked on the High Sierra update, so I’m back to Sierra which is now running on the SSD side alone. I have tried and retried all of the suggested software fixes to no avail. It’s definitely a hardware problem. I have purchased an external Thunderbolt/USB3 HDD to serve as storage or it may serve as the boot disk if needed.

My question is, can the external HDD and SSD be paired to serve as a Fusion drive once more? A visit to a repair shop is not practical given our very rural location so I’m grasping at anything that will not require the iMac come apart.