This is a sample plugin with code on how you can add your custom field type to existing field type to WooCommerce Easy Checkout Field Editor.
WooCommerce Easy Checkout Field EditorWooCommerce easy checkout field editor comes with following hooks which can be used to extend functionality in frontend and backend.
pcfme_override_field_types - to add new field type to existing field types
pcfme_after_visibility_content_tr - add custom field type option field to be shown after visibility dropdown