Assuming you are running, or plan to run, all your products using the S2 Sonos App, it’s perhaps important to note that only some of the old Sonos ‘Connects’ are compatible with S2 - those manufactured after 2015 (gen2), and those before are not. So any earlier ‘Connect’ device will ‘drag’ all your products back to the older Sonos legacy S1 App - so I would check and make sure with the seller that the advertised ‘Connect’ is S2 compatible otherwise I would personally go for the Sonos Port. See this link;

https://support.sonos.com/s/article/4786

Alternatively you ‘might’ find a solution with the TV that you have attached the Beam to. If that TV has an analog audio line-in, then you could use that connection to attach the turntable to, but you will find you need the TV powered on for playback to the attached Beam and there ‘may’ prove to be audio-sync (echo) issues when grouping the Beam with your Sonos one speaker.

If you want a temporary solution you could buy a cheapish RCA analog to digital optical converter and a digital optical switch. 

Run digital optical from the TV and from the ADC to the switch, optical from the switch to the converter that comes with the Beam.