can any stereo receiver be turned to mono?

If what you want is a stereo/mono switch to summing the stereo signal to mono, insert this circuit to tape-monitor loop, the tape-monitor button will become a stereo/mono switch.
https://www.vinylengine.com/turntable_forum/download/file.php?id=11690&t=1

Or use a stereo to mono converter box.
https://www.amazon.com/OSD-Audio-SMC21-Converter-Isolator/dp/B07FFBJ8KR/ref=pd_lpo_328_img_1/136-081...

Even easier, connect the + from both channels into one. +L and +R both go to chassis ground, so connecting L+R positive sums them yielding mono. This is what happens inside the Y connector. Either that, or the amp goes up in smoke. Which this being a receiver and knowing how I feel about receivers just may be my ulterior motive.

Only one way to find out for sure. 😂🤣

millercarbon - please do NOT give engineering advice that looks serious.   I understand that you might think this is a joke, but somebody may take your advice and potentially fry their receiver/amp.  You never want to connect two outputs of anything into a single target, especially if they are speaker outputs from an amp/receiver. 

Also, this is NOT the same thing as a Y connector.  It is perfectly fine to use a Y-splitter to send a single output to multiple inputs.  However, a special circuit is required if you want to "sum" multiple signals.

Just wanted to add, that for me mono means ONE loudspeaker playing a mono source, not summed stereo with two loudspeakers. OP, are you just trying to get mono to play on both channels? If that is the case, you can feed a mono source to one channel and connect both loudspeakers to that channel. You'll loose 3db of power but it will work.   

One more time. How DO you feel about receivers?

On the record I don't care for mono.  Both ears still work.  Kinda.