Hi,
What you are referring to is orphaned pages. These types of pages offer no SEO value as search engines can't find them. So my advice to you would be, if you want these pages to rank and can't add the link to the main navigation then add the link to the html and xml sitemap and if possible within body copy of your website (search engines should still find your pages). If you are building these pages just for Adwords and other advertising and you don't want search engines to find them then use noindex meta tags on each of the pages - https://support.google.com/webmasters/answer/93710?hl=en
Good luck,
Davinia