Connecting rPi 3 B+ & HiFiBerry DAC+ Pro to a radio's AUX IN?

I've got a Raspberry Pi 3 B+ and HiFiBerry DAC+ Pro on order.

I've got a SB Touch that's working well with my current audio setup.

Now I'm considering buying a pair of Audioengine A2+ speakers to pair with the Raspberry pi that will be used in a small room. But until I make up my mind about speakers, I'm thinking of using an RCA to 3.5mm cable to connect the HiFiBerry DAC+ Pro to the AUX IN of a Tivoli Audio Model One radio.

I know the sound won't be so good, and it's a waste of the DAC's capabilities, but I can live with that for now provided that the connection I'm proposing between the pi and Model One won't do any harm to any of the components. I can't see that it would, but I figured I'd ask for your expert advice. (Also, will what I'm proposing to do automatically convert what would otherwise be a stereo sound to mono sound?)

Thanks in advance for your advice and expertise.

Steve