Plugin for adding any field as an attribute to the WooCommerce CSV import

example-usage.php

<?php

function hwid333_add_custom_attribute_columns( $fields ) {
  
  $fields[] = array(
    'slug' => 'shirt-size',
    'name' => 'Shirt Size', // This is the name of the column in the CSV
  );
    
  $fields[] = array(
    'slug' => 'neck-style',
    'name' => 'Neck Style', // This is the name of the column in the CSV
  );
  
  return $fields;
  
}

add_filter( 'hwid_wooimport_attribute_fields', 'hwid333_add_custom_attribute_columns' );

woo-csv-import.php

<?php
/**
 * Plugin Name:     Woo CSV Import
 * Plugin URI:      http://hereswhatidid.com
 * Description:     Sets up the custom attribute options from a properly formatted CSV
 * Author:          Gabe Shackle
 * Author URI:      http://hereswhatidid.com
 * Text Domain:     woo-csv-import
 * Domain Path:     /languages
 * Version:         0.1.0
 *
 * @package         Woo_CSV_Import
 */

class HWID_Woo_CSV_Import {

	static $index = 1;

	public $fields = array();

	public function initialize() {
		add_filter( 'woocommerce_csv_product_import_mapping_options', array( $this, 'add_column_to_importer' ) );
		add_filter( 'woocommerce_csv_product_import_mapping_default_columns', array( $this, 'add_column_to_mapping_screen' ) );

//		Process the CSV data prior to running it through the Woo importer
		add_filter( 'woocommerce_product_importer_pre_expand_data', array( $this, 'set_import_object' ) );

//		This is just to allow for duplicate SKUs, for some reason the CSV import always throws a duplicate SKU error
		add_filter( 'wc_product_has_unique_sku', '__return_false' );

		add_action( 'init', array( $this, 'set_fields' ) );

	}

	public function set_fields() {
		$this->fields = apply_filters( 'hwid_wooimport_attribute_fields', $this->fields );

		foreach( $this->fields as &$field ) {
			if ( empty( $field['slug'] ) ) {
				$field['slug'] = sanitize_title_with_dashes( $field['name'] );
			}
		}
	}

	/**
	 * Register the custom columns in the importer.
	 *
	 * @param array $options
	 * @return array $options
	 */
	public function add_column_to_importer( $options ) {

		// column slug => column name

		foreach( $this->fields as $field ) {


			$options[$field['slug']] = $field['name'];
		}

		return $options;
	}


	/**
	 * Add automatic mapping support for custom attribute columns.
	 *
	 * @param array $columns
	 * @return array $columns
	 */
	public function add_column_to_mapping_screen( $columns ) {

		foreach( $this->fields as $field ) {
			$columns[$field['name']] = $field['slug'];
		}

		return $columns;
	}


	/**
	 * Processes the custom columns for inserting in to the custom attributes
	 *
	 * @param  array $data Data to import.
	 *
	 * @return mixed
	 */
	public function set_import_object( $data ) {

		foreach( $this->fields as $field ) {
			if ( ! empty( $data[$field['slug']] ) ) {
				self::add_attribute_group( $field['name'], explode( ', ', $data[$field['slug']] ), $data );
			}
		}

		return $data;
	}

	/**
	 * Creates the properly formatted array keys for the specific attribute set
	 *
	 * @param string $name - Full name of the attribute type
	 * @param array $value - Array of attribute values
	 * @param  array $data Data to import.
	 */
	static function add_attribute_group( $name, $value, &$data ) {
		$data['attributes:name' . self::$index] = $name;
		$data['attributes:value' . self::$index] = $value;
		$data['attributes:visible' . self::$index] = true;
		$data['attributes:taxonomy' . self::$index] = true;

		self::$index++;

	}

}

function hwid_woo_csv_import() {

	global $hwid_woo_csv_import;

	if( !isset($hwid_woo_csv_import) ) {

		$hwid_woo_csv_import = new HWID_Woo_CSV_Import();
		$hwid_woo_csv_import->initialize();

	}

	return $hwid_woo_csv_import;

}

// initialize
hwid_woo_csv_import();

Insert the code from 'sample-usage.php' in your theme or functionality plugin. The 'woo-csv-import.php' file is a standalone plugin.