You could use the REST API to create the blogs (use the /nodes/type/blog/add call), create the blog posts (use the /boards/id/<blog id>/messages/post call), and blog post comments (use either the /messages/id/<unique message id/reply call or the /boards/id/<blog id>/messages/id/<board-specific message id>/reply call).
In your REST calls to create the blog article and replies, you can pass the "message.author" parameter to post the message as another user (you need to make the REST call as an admin user who had the "switch user" permission). You can also backdate the message using the "message.post_date" parameter (I think you need to make the REST call as an admin user to do this as well).
If you have Wordpress blog categories you would like to carry over, you can include them as labels (the Lithium analog to Wordpress categories) -- when you make the messages/post call, just include your categories in the "label.labels" POST parameter.
Hope that helps!
-Doug
