“Convert” isn’t really what you are doing.  Nothing can magically convert an older device into a Bluetooth receiver.  What it will allow you to do is group older devices with the Bluetooth capable device, effectively sharing the Bluetooth content in multiple rooms.

https://support.sonos.com/en-us/article/share-bluetooth-audio-with-your-sonos-system

If the Play:5 has a line-in input you could plug in a Bluetooth receiver and input to that, without the need for an additional speaker. 

Thank you so much  jgatje  for your really helpful reply and link which is much appreciated. I think it was a figure of speech not being such a techni myself but I totally get what you’re saying. Many thanks for helping me. Christine  

hank you so much  jgatje  for your really helpful reply and link which is much appreciated. I think it was a figure of speech not being such a techni myself but I totally get what you’re saying. Many thanks for helping me. Christine  

Thank you nik9669a, I really appreciate your response and think that will be the easiest and cheapest option.  Didn’t really want to pay out for another speaker as mine have such good sound. Reason I need this option is that we don’t have fibre in our area yet and our WiFi can be sketchy at times. Many thanks. Christine 

Thank you nik9669a, I really appreciate your response and think that will be the easiest and cheapest option.  Didn’t really want to pay out for another speaker as mine have such good sound. Reason I need this option is that we don’t have fibre in our area yet and our WiFi can be sketchy at times. Many thanks. Christine 

No problem. I don’t know if you’re on S1 or S2, or what compatibility issues you might face with the “add a speaker” option, but the Bluetooth receiver seemed to me to be the simplest option. Note too that the line-in will work for any suitable source so if you have a cd player with suitable output, or an iPod, etc that too could plug into the Play:5.