Yes, add them as customers, then remove them when you don't need them any more.
I think there's a deeper use-case than you've described here though. Could you expand on that? My answer only caters for what you've asked, and I think there's more to it.
I was using "customer" in the sense of Jira account type.
These potential customers need access to your service desk so they can open requests for questions, get responses and so-on. Which is exactly the same as your real customers usage.
So you might as well add them as (Jira SD) customers, and remove them when you don't need them any more.
You might want to put them in an organisation together so you can identify them, and you might not want them to have access to the same portals as your real customers (until they become one), but that's a matter for the portals, not the account type.
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