Does this device have some sort of line level loop, such as a tape output/input? If so, you could easily connect it to a Sonos Port, without doing anything harmful to it. That would allow it to both feed the Sonos system with anything it plays, and play anything from the Sonos ecosystem as well. 

I don’t have it with me, but I believe the only built in input options are for radio antennas.

Then a non standard ‘hifi’ device….or old enough not to have more modern features. There’s not much of this community that does modding, from what I’ve seen. There are a few, or maybe just one third party company that installs Sonos in a console like device. I know I’ve looked at a website before, but a faint memory suggests these were ‘new’ consoles, and not retro fit devices. 
 

Without having line level outputs, things become significantly more complex. I’d recommend looking in other venues for possible communities that are more ‘in tune’ with what you’re trying to do, but I’m not familiar with any specifically. I’d avoid using the specific name of the console you’re trying to retrofit, and look for anyone doing similar work on any old device, as it would likely translate well.