IMAC(2015) is it possible to connect it to my stereo (in another room) so I can listen it on my good speakers?

Yes. Does the Macintosh have an optical audio input? That would be ideal. Otherwise you can use any 3.5mm stereo audio cable. Connect it to the Macintosh preamp. Impedance is not a factor.

I wouldn't write off the ability to use AirPlay so quickly. Of course you can't be certain unless you try it. If you don't want to risk spending money on a brand new AppleTV you can find discontinued Apple AirPort Express models readily available on the used market. The only AirPort Express models to avoid would be the very old A1084 and A1088 models. Don't buy them. Later models A1264 and A1392 will work fine. Their audio outputs are identical to the iMac's — both analog and optical.

Older, discontinued AppleTV models will also work fine for that purpose, and are readily available on the used market. Same audio output options. Newer ones are HDMI only.

Use AirPlay to stream audio - Apple Support

Please refer to About the audio ports on Mac - Apple Support

You will likely need some type of adapter however without knowing the input the Macintosh pre-amp has we cannot tell you what type you may need. For example, if the Macintosh pre-amp has an Optical Input then you can find an adapter fairly easily.

If the pre-amp is part of a Home Theater then I'd recommend buying an Apple TV 4K and stream your music from the iMac to the HT setup. I stream movies all the time from my iMac to my Home Theater system and also use the Apple TV 4K for streaming video content from the Internet. If you are not familiar with the Apple TV 4K please click the TV & Home link at the top of this page and then select Apple TV 4K.

You could maybe try to test some other Bluetooth device to see if you can get it to work from the same rooms through that wall. If some other Bluetooth device works, then you might have luck with a Bluetooth audio receiver like this: (they say it has a range of about 30')

https://www.bestbuy.com/site/aluratek-istream-universal-bluetooth-audio-receiver-black/6581843.p?skuId=6581843

If going the hardwired option, and if your wire run is very long, like more than 20 feet or so, roughly speaking, then you won't have much luck running RCA cables that far either, you may get too much signal loss. You could try it, the headphone output and the pre-amp input might be good enough to handle it, even with a 50' cord possibly, again it just depends. An XLR cable, typically used for microphones, can run audio signals for much longer distances, but you need something like an impedance adapter and/or DI box and a separate cable for each stereo channel.

I almost wonder if you can move the stereo closer to the computer for a hardwire option, and use longer speaker cables to the speakers in the other room?

You need one of these

Picture is from https://www.cablesdirect.co.uk/av-cables-adapters-c477/audio-cables-c478/audio-c479/3-5mm-stereo-to-twin-rca-cable-p1898 and the length is probably inadequate but they are very common.