I’d add whichever device suits you with a line in. There are several, but once you get the data in to the Sonos system, you can play that in any room. All analog line ins are equal, on Sonos, there isn’t one that is different/better than others. 

A Victrola Stream will link to your current Sonos setup.

Look here, buy where you get the best support and deal.

https://www.crutchfield.com/g_10500/Turntables.html?tp=75887&fa=1#&nvpair=FFBrand|Victrola

The Project T1 Evo phono is an 'audiophile’ entry level turntable with a built in phono pre-amp, replacement for the excellent T1 SB. Comes with a factory fitted Ortofon OM10 cartridge, easily upgradable to an OM20. If budget allows the Debut Evo 2 is one of the best in its price bracket but needs a separate pre-amp. If you get into turntables, the Debut’s anti-skate is something you will appreciate. Automation is not something I’ve ever looked for in a turntable in 45 plus years, rather the money was spent on things that affect the audio quality. Sonos don’t tell us what DACs are used in what speakers/devices, but I noticed a difference between connecting my turntable through an old Connect versus an Era 300, probably to be expected giving the age difference between the two.

As @Stanley_4 says, the easiest option is a Victrola Stream model which on its own can connect wirelessly into your existing Sonos system. 

As you’ve realised, when operating as surrounds, what ever speaker you use there, it can’t use any additional inputs it may have.

So you have a few options:

• Add a Sonos Port with the Pro-ject SB, and then just group it with the existing setup to play records. 
• Add another speaker with a line in, but then that speaker will also play unless you mute it, in which case the Port is probably the neater option.
• Go a bit more off-beat and replace the Arc with a Sonos Amp and a pair of passive speakers at the front. You can still have surround audio for TV/movies, but the major benefit here will be you can easily disable the surrounds with this configuration and listen to your records in proper stereo at the front… worth considering if you think you are going to get into your vinyl. With the Arc, the surrounds are permanently enabled and you can only control whether they are full or ambient. I’m not sure either how good the Arc would be for stereo only listening compared to a pair of ‘normal’ passive speakers. 

Thank you all for the opinion, I think getting a port would be the most easy way for me. 

Line-In does not play on the hosting speaker by default. Line-In is a resource on the network that can be played on any combination of speakers, but only when selected by a given speaker. While the controllers are aware of which speaker is hosting the Line-In Jack, this is simply a convenience for the operator — useful if there are multiple Line-In’s active. Line-In ‘A’ can play only on speaker ‘B’, only on ‘A’ Grouped on ‘ABCDE…’ as selected by the user.