Allow admins to manually enter new WooCommerce orders from the frontend checkout field.

admin-create-new-customer-on-frontend-checkout.php

<?php

// Allow admins to manually enter new WooCommerce orders from the frontend checkout field.

// Hook the woocommerce_checkout_customer_id filter found in the WC_Checkout class. 
add_filter( 'woocommerce_checkout_customer_id', 'change_current_user_id_to_new_user' );

function change_current_user_id_to_new_user( $user_id ) {
    //Check if logged in user is admin.
    if ( current_user_can( 'manage_options' )  ) {
        // grab the $_POST values from the checkout form needed to create a new user.
        $checkout_email = $_POST['billing_email'];
        $first_name = $_POST['billing_first_name'];
        $last_name = $_POST['billing_last_name'];
        // Check if an exisiting user already uses this email address.
        $user = get_user_by( 'email', $checkout_email );
        // If the user email already exists then pass along their user ID, otherwise create a new user.
        if ( $user ) {

            $new_user_id = $user->ID;

        }else{
            
            // Randomly generate a password.
            $password = wp_generate_password( 12, false );
            // Use the email for username and email to avoid conflicts.
            $new_user_id = wp_create_user( $checkout_email, $password, $checkout_email );
            // Add a first name, last name, and the customer user role to the new user.
            wp_update_user( array( 'ID' => $new_user_id, 'first_name' => $first_name, 'last_name' =>  $last_name, 'role' => 'customer' ) );

        }
        // Make sure nothing went wrong.
        if ( !is_wp_error( $new_user_id ) ) {

            $user_id = $new_user_id;

        }

    }
    // Be sure toreturn the user ID.
    return $user_id;

}