You can but then wouldn’t you technically have 2 landing pages on one page? Were you planning to somehow hide the old landing page content as to keep the SEO value but only display the new landing page?

Maybe it would be easier to just build links to that new landing page and make sure that you are keyword targeting correctly. Also create a xml sitemap for the web page and submit it to the search engines so that it will be crawled easier.

By doing the above and using optimized meta info, you will get a increase your chances of getting a good SEO position for your leaner landing page.

You can use an iFrame to put your Unbounce page at the beginning. Just be sure not to hide your old content. Also, using an iFrame can complicate issues for tracking, but if that isn’t a problem it will work.