There is no tested work around for using the captcha on an ASP page. The captcha depends on a Session variable being set with the captcha answer to compare to the entered form value, and I don't think you can refer to a php session from an asp page.
You might be able to create another PHP page to return the session variable value and then use that for comparison, but I'm not even sure how that would be done. Another solution might be to use AJAX to check the value by posting to the php page.
Either way it will take some work, but is probably possible but not straightforward or easy.