Hey Brandt,
We aren't using SDFC cases at my current company, but I used to handle cases all the time in a previous role. The first thing to figure out is whether SFDC cases are a record type that is automatically sync'd to Marketo (hopefully that's how it works) or a custom object that you have to manually sync.
Either way, once you've resolved the sync questions, you should be able to use something like "Added to SFDC case" as a trigger in a smart campaign, which then adds the related record to a campaign in the flow step.
-patrick