Is it possible to add a portfolio page to the nav bar?

I can't find a way to do this and really want to avoid having to create a new page from scratch just so it can feature in the navigation bar. I'd be hugely grateful for any guidance!

This should help you:

https://support.squarespace.com/hc/en-us/articles/205814778-Adding-pages-to-your-navigation

If it is an existing page or section and not on the nav bar you can add an anchor link:

https://support.squarespace.com/hc/en-us/articles/207135178-Creating-anchor-links

Thank you Derrick, really appreciate your help!