You cannot use the "Fusion Drive" setup externally.
If the hard drive is still good you can install it into an enclosure, but I would expect the hard drive is starting to wear out and getting near to failing by now. You can check the health of the hard drive by running DriveDx and posting the report for the hard drive here using the "Additional Text" icon which looks like a piece of paper.
If the SSD part of the Fusion Drive is large enough you could see if OWC has an Envoy Pro Enclosure that is compatible with this SSD.
Are you installing a PCIe SSD or a 2.5" SATA SSD? If the latter, then you could leave the original Apple SSD installed (do not recreate the Fusion Drive using two SSDs).