Sorry to see that this hasn’t been addressed yet. This just reminded me of an issue that someone else is having, and an idea came to mind that I just shared with them, and which I’ll also share with you.
One possible alternative (if you’re willing to explore a bit) is to consider setting this up in Integromat. I don’t know enough about Zapier to know if there’s an equivalent for this, but Integromat has a Flow Control module with an Iterator operation that’s designed for just this type of situation. In short, it’s designed to take an array—which is probably how the usernames in your situation are being fed to Zapier—and split it into individual items, which are then processed one by one by the rest of the scenario. With this, your usernames could be added individually to create a series of Airtable records.
I tried searching for something similar in Zapier for iterating through a collection, but couldn’t find it. If you know what it is, you could probably do this same thing in Zapier. If not, I seriously recommend giving Integromat a try. I’d be happy to help if needed.