How to make “Write Page” a real link outside of WP?

I’m using WP as kind of a “front page” for a company. I’m still a Wp newbie, but the basics are working fine.

I want to make a “page” link over in the side bar, that
is actually a link to a whole different section of the site that isn’t maintained under WP, but a different system all together.

Do I have to edit the sidebar php directly? Because I’d like to be able to let the customer maintain that link inside the WP system, w/o having to turn them loose editing php or anything…

I appreciate the help.