Looks like you're wanting to have a feature akin to an Activation Key used in licensing downloaded software...
Currently, there's no feature (or bolt-on module) in Zen Cart that performs that manages this (rather detailed) process.
But why are you wanting to create a separate password???
If you create a downloadable product, when a customer purchases that product, your configurations "point" the customer to the download... and ONLY your customer will have access to that download. (The .htaccess control in the DOWNLOADS folder prevents unauthorised people from getting in there.)
If (once the download has been installed on the client's computer), they ALSO require an activation password, you can do this manually when you manage their orders.
... then, using the "status and update" feature (bottom/left of order management screen), you could insert the password into the comments field, update the status, and get zencart to send the status message to the customer. That comment is also available in the customer's "My Account" file, so if they lose the password, they can log in to your shop, go to My Account, and retrieve it.
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