I believe that the editor should do this naturally, extending the page.
Something that might be making it not work could be the template element with a fixed height. If you make the height bigger, it will extend the page down further (below the fold). The heights to check:
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BODY#bodyId
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DIV#mktContent
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DIV#innerWrapDiv
You can click on each of the above to select it, then click on the "Property Sheet" little dropdown in the top-right corner of the landing page editor. From there, you can change the "height" in pixels to make the page longer.
In terms of editing templates to achieve this, just make sure that there is no stated height of any of the above three.
Best,
Edward Unthank
SEO/Web Specialist