This one’s pretty simple luckily. To add a static page to the navigation bar (the term ‘static’ is misleading because all pages are technically content managed in Magento) the best way is to create a ‘Static Block’ in CMS/Static Blocks. Make this your new page and add the content you want.
The create a new category and set the Display Mode to ‘Static Block Only’ under Display Settings – then choose your new static block as the destination page.
As long as it’s active, this should now appear in the navigation bar.