You won’t be able to use the Yoast plugin on your main site without re-designing or importing the site into WordPress.
In some instances, you could use some wordpress features for portions of your site if you design a wordpress site that looks identical and run it along side. This is sometimes done when people run a store on a service like squarespace, but want a blog and other features via wordpress, but it’s probably not recommended.
What platform is the site currently running on?
There may be SEO plugins for the platform, or other solutions. Also it’s possible that via something like CMS2CMS, import could be straightforward (but don’t count on it).