Well, I guess it all depends upon what you consider to be "easy", but I don't know any way to do it "from a text file".
Sure, DreamHost allows you to create unlimited email addresses, but it is not in their, or other DreamHost users', best interests to make it easy for you to create 800 of them.
If you search these forums, you will see example working codes that sets up "forwarding only" email addresses via a PHP script ... I suppose you could modify that to suit your purposes if you are proficient in PHP and the use of cURL.
That said, should you do that, and use it to add 800 mailboxes, you may well find that DreamHost modifies their panel to preclude the use of such a script in some cases (as they have done by adding CAPTCHA to the "add a domain" page once you have "N" number of domains hosted). It seems the SEO tards where going crazy and creating subdomains "by the thousands".
I'm also wondering if you have though about the foo of trying to remain compliant with the DreamHost Anti-Spam policy while maintaining 800 email accounts? I, for one, wouldn't want any part of that potential grief.
This kind of volume just screams for the uses of a dedicated email provider, which you can do easily enough as DreamHost lets you modify your MX records to facilitate "off-host" mail handling (though the potential spam issues are still there)