Add USB-C ports to an old 2014 iMac
I am not sure if you have a 2014 or 2015 iMac, but this does not matter since the answer is the same. Both models have USB 3.0 which is 5 Gb/s. However, the jacks are Type A and you want Type C. The Type A and Type C plugs have different geometry, but electrically they are the same. See this answer for illustrations. Since Type C is reversible, there has to be a chip in the adaptor to properly transfer the signals.

For example, you can use this adaptor from Electop. You should look for adaptors where the tip is blue, otherwise you may end up limited to USB 2.0 speeds (480 Mb/s).

If you do not want the adaptor hanging on the back of your iMac, you could add an extension cable. They come as short as 6 inches.