Sorry, you cannot. Site Wizard will interpret any content you add to a text field as plain text, so any HTML tags you add won't be translated properly on your page. For basic text formatting, you can use the font style options available on each page. To format paragraphs, simply hit the Enter key on your keyboard to create a natural line break. You'll be able to add links of your own only on the "Additional Links" page.
To use your own HTML or take advantage of more advanced layout and design tools, you'll need to upgrade to a Yahoo! Web Hosting plan and edit your pages with a more powerful site building tool, such as Yahoo! SiteBuilder or Yahoo! PageBuilder.