How do i add slugs to a URL, but still redirecting to the same page

Unless there’s a reason the subcategory URL has to be www.webshop.com/slug/category1/subcategory1, I would modify this to use a hash instead – i.e. www.webshop.com/slug/category1#subcategory1

This way you won’t break WordPress’s permalink structure and you can still easily use the hash with JavaScript so render the correct content on the page.