You can add HTML code in both Topol PRO and Topol Plugin. By adding this code you will be able to use features such are the countdown timers and custom tags that are required for using dynamic content.
You can find this feature in your settings panel, under the tab content. You can simply drag it into your email template, and after the HTML block appears in the template a window with the code will appear in the settings panel.
Make sure you test your template well. Keep in mind that some features might look good in your template editor, but a mailbox could have trouble displaying them.