OK, so you want to display the link on pages outside Zen Cart as well as in a custom HTML link list in the ZC header. The simplest answer is to add a new link for the define page the same way you made the links for the other pages. Be aware that navigating out of ZC for a while can cause the session to be lost along with anything in the shopping cart.
Once you get the ZC template modified to look like your original page design, you can transfer all other content into ZC pages and keep the navigation "in the family".
You can use the ez-pages header bar, restyled to match and fit where your current links are, in place of the hard-coded links.