import (migration) user database to wp-users

1. get all data from DB1( non-wp-database ) and save to an $array.
2. Use this wordpress code inside your functions.php for insert users into DB2 ( wordpress database).

function cxg_wp_insert_user() {
    foreach ( $array as $arr ) {
        $user_data = array(
        'ID' => '',
        'user_pass' => wp_generate_password(),
        'user_login' => $loginName,
        'user_nicename' => $nicename,
        'user_url' => '',
        'user_email' => $eMail,
        'display_name' => $displayName,
        'nickname' => '$nickname,
        'first_name' => $firstName,
        'role' => get_option('default_role')
        }
    );
    $user_id = wp_insert_user( $user_data );
}
add_action( 'admin_init', 'cxg_wp_insert_user' );
 3. After updating disable this function

NOTE : For more details about insert user goto wordpress insert user codex