Here is a script you can run after the fact to open up commenting on posts imported by my script.

Place the script in your textpattern directory, rename it from open-comments.php.txt to open-comments.php, and run it by browsing to the page in your web browser. I’ve ran it on my install at home, and it seems to work fine. It will run the following query:
update textpattern
set annotate = 1;

This will open up commenting on ALL your posts. This script is meant for people who don’t have access to phpMyAdmin, or don’t want to run a SQL query themselves.