Seems disingenuous, but you can create multiple users with a single email account by using email addresses like this:
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.
If you already have the 1000 posts and 200 usernames created in some machine-readable format it would probably be worth a few minutes to look at how the API works and/or paying someone to do it for you, but then, I’m a programmer, so I would think that. Or course, I’d rather spend 20 hours writing code than 10 copy-and-pasting, because what if you ever needed to do it again?
You’re definitely right about social proof, but creating fake users is questionable practice and I’d strongly recommend against it, for the long term health of your community. There are more ethical ways of doing it (which won’t result in a backlash). If you want some advice around doing things differently, I’m happy to help.