Your Sonos Ones and Sub are bonded  together into a single Sonos room. A port can’t be bonded with any other Sonos device, it’s always a room all by itself.  However, any Sonos room can be grouped with other rooms to play music in sync. So yes, you can the Port to your Sonos sytem, you’ll have two rooms.  You don’t need to get a Five, but many fine it to be a better fit for their home since you get the speaker as well as the aux port.

To add to the above, you will set up the Port as a separate room (call it ‘Port’).  Then go into the setup for the Port and set Autoplay to ‘On’ and the default Line-In Room to be the room with the Ones/Sub.  That way, anytime you put the needle down, it will automatically start playing on the Ones/Sub.  

You could leave your Sonos Ones stereo paired and bonded with the Sub and add any Sonos product with a Line-in, such as a ‘Port’ or a Sonos Five for example, to which you can then attach a Turntable/pre-amp to output its line-level audio to the Sonos One room & sub.
 

Its just a bit of an expensive audio line-in for your system, so my personal suggestion would be to go with the Sonos Five speaker and make use of that (great) speaker somewhere in your Home and locate the turntable nearby (if that is at all practicable)?

Thanks for all the quick reactions, very helpful. And happy to hear that it is possible to combine the port this way. 
 

 I recon all that is said for the Port also holds true for the older Connect right?