Adding Individual Leads to a Smart Campaign?

You'll need to add the lead to the list and re-run the smart campaign, ensuring your campaign settings are set to only allow a lead to run through the flow once (this will prevent the rest of the list from getting the email again).

Another possibly easier way to do this sort of thing, if you need to do it a lot, is to always add the "Campaign is Requested" trigger to your smart campaigns (source= Marketo Flow Action). This allows you to call the campaign on an individual lead or another smart campaign. So, by adding this trigger, you could then find the lead in your lead database, right click, and under special actions, use the "request campaign" flowstep. Note that this is the same sort of process you'd follow if you wanted to allow your sales team to add individual leads to your campaigns (the only difference would be that you'd change the source to "Marketo Sales Insight")

Hope this helps!