Connect wireless Xbox 360 controller to iMac?

The absolute best way to use any sort of Xbox controller on Mac is to install the 360Controller unofficial open source driver:

https://github.com/360Controller/360Controller#about

It works with the official Microsoft 360 wireless receiver. Probably also the knockoffs. It works with tons of unofficial 360 controllers. Tons of Xbox One controllers. Even tons of original Xbox controllers, if you can figure out how to plug them in.

BUT! There is an issue with Xbox 360 wireless controller support on newer versions of OS X:

As of macOS 10.11, Wireless Xbox 360 controller support causes kernel panics. This issue cannot be resolved with minor changes to the driver, and requires that the driver be re-written from scratch to resolve the issue. Due to an excess of caution, we have disabled Wireless Xbox 360 controller support as of 0.16.6. If you want to use a wireless controller, download 0.16.15 or earlier and disable the driver before the computer enters a "sleep" state in order to prevent kernel panics. Alternatively, you can revert to a macOS version before 10.11 to avoid this issue.

Depending on how old your version of macOS is, this might work for you.