Unfortunately, the migration process does not preserve information on page and post authors. If this is something that you care about, you'll need to set the authors manually. However, you can make bulk edits on multipage pages or posts at one time:
Add any authors as users of your site (see Add Users section on this page). To be assigned as an author, the user must have a role of Contributor, Author, Editor, or Administrator.
Go to Pages > All Pages or Posts > All Posts
Using the checkboxes next to the page/post titles, select one or more pages/posts for which you'd like to assign to a specific author.
Select Edit from the Bulk Actions menu above the page/post titles.