Porting SW Radio Speaker Output To iMac: How ?

The iMac headphone out jack is just that, it is not a mic/audio input.

In order to input audio from an external source, you need an analog/digital converter (like the Behringer device you mentioned).

Regarding using a speaker output as a source, however, that's another issue you will have to deal with. Speaker output levels are too high & impedance too low - they do not match either line or mic input levels. You may need an additional device to convert from speaker level to line level or mic level.

Speaker out is typically in the 30-50 volt, 8 ohm range. Audio line input is nominally 0.03 to 1.0 volts and 100-600 ohms. Mic in is typically only 0.01-0.001 volts and 150-200 ohms (or as high as 2.2K ohms with certain mics). Hence the need to match your output to your input.

You will need to get a USB audio in device. There are lots available, at every price point.

You do need to be a bit careful, a speaker output can be much too high for a typical mic-in or even line-in port, it needs to be attenuated.

I'm an Advance class ham radio operator ... these guys pretty much wrote the book on ham radio and short wave listening: https://www.arrl.org/

If anybody is going to know how to do this, the ARRL will or maybe could point you to those who might.