Seems disingenuous, but you can create multiple users with a single email account by using email addresses like this:
my.gmail.username@gmail.commy.gmail.username+user1@gmail.commy.gmail.username+joe@gmail.commy.gmail.username+funnyguy@gmail.commy.gmail.username+smartypants@gmail.commy.gmail.username+guyWeHate@gmail.com
and so on. Most email systems support this convention.
There’s a bulk import feature, so you could create them all in a spreadsheet or whatever.
The API is a great took but you might also put the content in some database and write an importer to populate Discourse. See vanilla.rb and vanilla_mysql.db for examples. I just wrote an importer to import an image gallery. I thought it was a bad idea at first, because it would seem like an image gallery would be a good way to display an image gallery. But it turns out that the image gallery in Discourse is pretty freaking great.