Blogger does not supply the email addresses when you port the users across, they are all "firstname.lastname@example.org", so it is a bit difficult to identify all the comments linked to a specific commenter.
It might be able to match the users on the username and/or website but I'm thinking that for large sites the numbers could be excessively large.
If you just want to fix the gravatars it's possible to do that with a series of update queries, I use something like the following to fix up the missing emails on the comments I've made myself.
update wp_comments set comment_author_email = 'email@example.com'
where comment_author = 'Andy from Workshopshed'
You could do something similar to set the user_ids.