You can setup the playbar as a speaker in another room, yes.  Is that what you’re getting at?

In short, Yes. If you have two TV’s then both will be shown as separate Sonos Rooms and can be grouped together for multi room music play back.

What I am trying to do is update to the arc.

Then take my current playbar and put it in my office room, it will then only be used as part of a music system with the two play 5’s that are already in the office room and not physically connected to a tv. The office room setup would only be used for playing music

I would then have the new arc, a sub and two play ones in the lounge.

Yes that would work fine. You could add the Fives as surrounds. The only issue that I can see is if your Play: 5's are Gen 1 which would mean that you are stuck in a S1 App, with limited new functionality going forward.

If you have two play:5 in your office already, I don’t think you’re going to the audio for music by adding in a playbar.  If you don’t have another room where you could use speakers, I would look at selling your playbar, or giving it to a family member.   

Thanks for the comments.