iThemes Exchange attempts to create a custom customer/registration field.

company.php

<?php do_action( 'it_exchange_content_registration_before_company_element' ); ?>
<div class="it-exchange-registration-company">
	<?php it_exchange( 'dichiara_registration', 'company' ); ?>
</div>
<?php do_action( 'it_exchange_content_registration_after_company_element' ); ?>

dichiara.php

<?php

# this would be included in functions.php

require_once( 'dichiara_registration.class.php' );

add_action( 'after_setup_theme', 'dichiara_setup', 12 );

function dichiara_setup(){
	add_filter( 'it_exchange_get_content_registration_field_details', 'dichiara_itex_registration_fields' );
	add_filter( 'it_exchange_get_content_checkout/elements/purchase-requirements/logged-in/elements/registration/_fields_elements', 'dichiara_itex_registration_fields' );
	add_filter( 'it_exchange_get_content_profile_fields_elements', 'dichiara_itex_registration_fields' );
	add_filter( 'it_exchange_get_purchase_requirments', 'dichiara_itex_purchase_requirements' );
}

function dichiara_itex_registration_fields( $fields ){

	$new_fields = array();
	foreach( $fields as $field ){
		$new_fields[] = $field;
		if( $field == 'last-name' ){
			$new_fields[] = 'company';
		} elseif( $field == 'email' ){
			$new_fields[] = 'phone';
		}
	}
	return $new_fields;
}

function dichiara_itex_purchase_requirements( $requirements ){
	$requirements['phone'] = array(
		'priority' => 2,
		'requirement-met' => 'dichiara_itex_get_customer_phone',
		'sw-template-part' => 'phone',
		'checkout-template-part' => 'phone',
		'notification' => __( 'Phone number is required to checkout.', 'dichiara' ),
		'slug' => 'phone'
	);

	$requirements['company'] = array(
		'priority' => 4,
		'requirement-met' => 'dichiara_itex_get_customer_company',
		'sw-template-part' => 'company',
		'checkout-template-part' => 'company',
		'notification' => __( 'Company name is required to checkout.', 'dichiara' ),
		'slug' => 'company'
	);

	return $requirements;
}

function dichiara_itex_get_customer_phone( $customer_id = false ) {
	$phone = it_exchange_get_customer_data( 'phone', $customer_id );
	return apply_filters( 'it_exchange_get_customer_phone', $phone, $customer_id );
}

function dichiara_itex_get_customer_company( $customer_id = false ) {
	$company = it_exchange_get_customer_data( 'company', $customer_id );
	return apply_filters( 'it_exchange_get_customer_company', $company, $customer_id );
}

dichiara_registration.class.php

<?php

if( ! class_exists( 'IT_Theme_API_Registration' ) || class_exists( 'IT_Theme_API_Dichiara_registration' ) )
	return;

class IT_Theme_API_Dichiara_registration extends IT_Theme_API_Registration {

	/**
	 * API context
	 * @var string $_context
	 * @since 0.4.0
	*/
	private $_context = 'dichiara_registration';


	/**
	 * Constructor
	 *
	 * @since 0.4.0
	 * @todo get working for admins looking at other users profiles
	 * @return void
	*/
	function __construct() {
		$this->_tag_map['company'] = 'company';
		$this->_tag_map['phone'] = 'phone';
	}

	/**
	 * Outputs the customer's phone data
	 *
	 * @since 0.4.0
	 * @return string
	*/
	function phone( $options=array() ) {
		$defaults      = array(
			'format'      => 'html',
			'label'  => __( 'Phone Number', 'it-l10n-ithemes-exchange' ),
		);
		$options = ITUtility::merge_defaults( $options, $defaults );

		$field_id = 'phone';
		$field_name = $field_id;

		switch( $options['format'] ) {

			case 'field-id':
				$output = $field_id;

			case 'field-name':
				$output = $field_name;

			case 'label':
				$output = esc_attr( $options['label'] );

			case 'html':
			default:
				$output = '<label for="' . $field_id . '">' . esc_attr( $options['label'] ) . '</label>';
				$output .= '<input type="text" id="' . $field_id . '" name="' . $field_name . '" value="" />';

		}

		return $output;
	}

	/**
	 * Outputs the customer's company data
	 *
	 * @since 0.4.0
	 * @return string
	*/
	function company( $options=array() ) {
		$defaults      = array(
			'format'      => 'html',
			'label'  => __( 'Company Name', 'it-l10n-ithemes-exchange' ),
		);
		$options = ITUtility::merge_defaults( $options, $defaults );

		$field_id = 'company';
		$field_name = $field_id;

		switch( $options['format'] ) {

			case 'field-id':
				$output = $field_id;

			case 'field-name':
				$output = $field_name;

			case 'label':
				$output = esc_attr( $options['label'] );

			case 'html':
			default:
				$output = '<label for="' . $field_id . '">' . esc_attr( $options['label'] ) . '</label>';
				$output .= '<input type="text" id="' . $field_id . '" name="' . $field_name . '" value="" />';

		}

		return $output;
	}

}

field-markup.html

<!-- company field -->
<label for="company">Company Name</label>
<input type="text" id="company" name="company" value="">

<!-- phone field -->
<label for="phone">Phone Number</label>
<input type="text" id="phone" name="phone" value="">

phone.php

<?php do_action( 'it_exchange_content_registration_before_phone_element' ); ?>
<div class="it-exchange-registration-phone">
	<?php it_exchange( 'dichiara_registration', 'phone' ); ?>
</div>
<?php do_action( 'it_exchange_content_registration_after_phone_element' ); ?>