Yes. You can create an HTML file named 'addon_content_additionalform.wws' and store it in your templates folder.
You can also add the page 'ProfileCheckoutOnePage_v1_CustomLeftBlock.wws' which will insert content on the left column of the checkout page.
You can also add the page 'ProfileCheckoutOnePage_v1_CustomContent.wws' to add additional content to the checkout page.
These files should follow the same CSS classes that you find on the one-page checkout form. View source on the checkout page to see what CSS classes are available.
