How to add one artboard into a Flow I have already set up?

A little puzzled by this thread. You can of course add artboards to an existing flow. Then you'll need to re-wire the interaction links to incorporate the artboads added into the flow.

I had this same issue, and all the answers here seemed to either miss the point, not work for me, or the instructions were unclear. After some reading and poking around I think I've figured it out, so thought I'd share.

1.  In Prototype mode, use interaction wires to connect your new artboard to at least one in the existing flow.

Note: If you have a series of standalone wireframes grouped into a flow for ease of sharing with stakeholders, but they're not intended or set up to be navigated between, you may find that while the connections between your existing artboards are blue, your new one is grey. Don't worry, it should still work, but if you want to fix this, you've most likely just connected something ON the existing artboard to the new one, as opposed to the artboard itself. Get rid of your grey line, select the name of the artboard you want to connect from (the whole thing should highlight blue) and reconnect to the new artboard. Voila! It's ok, I have OCD too. 🙂

2. In most instances for me, this already successfully linked the new artboard into the existing flow. For flows with a link set up, however, I sometimes found I had to update the link before it would recognise the new artboard as part of the flow.

This makes no sense.  It means that everytime I make changes, I have to publish a new link.  So there's piles of links and reviewers get frustrated.  For a user experience tool, this is a poorly conceived user experience.