Hi,
It depends...some adapters/multiple adapters don't pass power through the connection.
-What brand/model is the external SSD drive/enclosure...Samsung?
-Is it a built-in USB-C connector, or removable?
I would think you would have better luck finding an adapter that would work to connect to one of the USB ports rather than the Thunderbolt connection on the iMac. And, if you do find an adapter to connect, you probably won't get the full speed of the SSD/USB-C connection, due to the older, slower connections on the iMac.
C