Hey Nick, it really depends on how you have the campaign set up. Assuming you're adding new people into the same static list, you would want to make sure that you have a criteria on the Schedule page set to only allow people to flow through the Smart Campaign one time. If you're adding a completely new static list, you will need to change the smart campaign smart list to account for the new static list.
If you have it set to send one email, wait 2 weeks, then send another email, anyone who is in the wait step when the smartlist criteria changes will remain in the flow.
If you're newer to using Marketo, I'd probably suggest cloning your original smart campaign and making changes in the new smart campaign, to ensure that you don't erroneously mess things up for people who are currently in the wait step.
Feel free to reply with screenshots if this doesn't answer your question, and you think it would help provide more insight into what you're trying to do.
But also, since I didn't mention this, simply adding a new static list, or even adding people to a static list won't necessarily cause them to go through any flows. You need to have a smart campaign that is actually causing some type of action either due to a trigger, or a batch campaign.
+1 on Dory.
Another way to think about it is like this:
Yes, if the batch were recurring and the new Static List was added to the Smart List after the First Batch Run (or new people added to the Static List after the first run), then yes, the new people would be added (if they qualify other filters) to the top of the flow.
Now, if you add to Static List that is a condition within the Flow, and those new people never qualified for the original (or previous) batch runs, those NEW people will NOT have any actions taken within the flow.
New people must be added to the top of the flow for anything to happen to them. There are no "in flow" insertions.