I have an iMac, late 2013, 27". I want to run a USB hub from the Thunderbolt port. Is this possible?

Thunderbolt 2 docks and hubs are very expensive.

https://www.amazon.com/Elgato-Thunderbolt-Dock-Ethernet-aluminum/dp/B00PAPMSLE

Your best bet is to just get and use a USB 3.0 hub.

https://www.amazon.com/dp/B08926X7LP/ref=sspa_dk_detail_2?

This adapter by Apple should do the trick depending on the type of Thunderbolt port you have: USB-C to USB Adapter

However why not use a USB hub on one of the USB ports (a self powered hub should be used). That way you wouldn't need an adaptor at all.

Your Late 2013 iMac has 4 USB 3.0 ports and 2 TB 2 ports. The TB 2 ports support Displays, TB 2 Drives and USB, but only thru a limited number of those expensive TB 2 docks.

If you need more USB 2.0 or USB 3.0 ports, then all you need is a USB 3.0 hub.

Since your computer was made Apple changed the size & shape of the thunderbolt port. I've just bought a 2020 iMac and it has the newer Thunderbolt ports. The key take-away now is you can go to Amazon and purchase a pair of adapters that plug into the thunderbolt ports and have USB female on the other end. They are made by Syntech and a pair costs only ten dollars. Search for Syntech USB C to USB Adapter (2 Pack), Thunderbolt 3 to USB 3.0 Adapter Compatible with MacBook Pro 2019 and Before, MacBook Air 2020, Dell XPS and More Type C Devices.

USB3 A fits USB2 sockets.