Add USB-C ports to an old 2014 iMac All my Apple devices are fairly new and apart from Lightning cables, all my peripherals are now USB-C. 

But for photo editing I still like to use my 2015 27" iMac that was heavily upgraded. Now this iMac only has two Thunderbolt 2 ports and, obviously, no USB-C ports. I would like to add a hub to my iMac that allows me to use USB-C devices, like hard drives that are mostly just USB 2 or USB 3 anyway. A single USB-C port would be enough. At the moment I keep a spare set of USB-A to whatever cables around but it's getting increasingly annoying.

Is there a Thunderbolt 2 dock that would have a USB-C port or a way for me to use something like Satechis USB-C hub on my Thunderbolt 2 iMac? It does not need to be a Thunderbolt 3 port or support PD, all of which would require an active component.
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.