Adding metaboxes to user profile - WebDevStudios/Custom-Metaboxes-and-Fields-for-WordPress GitHub Wiki

To add custom metaboxes to the user profile page, you can set the pages variable to user to indicate that the metaboxes should be displayed on the user profile (rather than a specific post type) and that the meta information should be stored against user_meta, rather than post_meta.

An example metabox can be seen in example-functions.php