There is no Fusion drive as an individual disk. It is a regular HDD used in conjunction with an SSD located on the computer's motherboard. So, the simple answer is you can put the HDD into any external enclosure that will connect to your computer. You can find suitable enclosures at OWC or Best Buy.
The SSD would be installed where the HDD was installed. You will need a special connector for the temp sensor or the fans will run full speed forever. You can find quality SSDs plus the connector cable you will need at OWC. They also have tools you may need to disassemble your iMac as well as instructions for disassembly, installation of the SSD, and reassembly.
You probably don't care but you should know that once you open the computer Apple will no longer provide any service. Hope you don't have a problem requiring parts only available from Apple.