How to connect a usb c external ssd to a thunderbolt 2.0 port on a 2013 iMac

You can not use the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter in reverse, because it does not supply power to the external SSD drive in that direction. About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support

You can use a USB-A to USB-C adapter in one of your iMac's USB-A ports.

Just keep in mind that the External SSD will only run at the slower USB speed.