You'd have to customize the template where that appears and use code for generating the states drop-down in place of that field, something like this:
PHP Code:
{include file="main/states.tpl" states=$states name="address_book[`$id`][state]" default=$address.state|default:$config.General.default_state default_country=$address.country|default:$config.General.default_country country_name="country" id="state`$id`"}
The template might be main/register_additional_info.tpl