Creating a staged user with an API call

I’m trying to avoid creating a full-fledged user account and picking a username. Essentially I want to replicate whatever is happening in the email trigger (‘custom incoming email address’ in the group settings) where the sender’s email address will be ‘staged,’ such that if/when they do register, they’ll be able to claim any messages that were created on their behalf via the email trigger.

Does that make sense? Is there a way to ‘spoof’ an incoming email using API calls? Thanks for all your help!