Next/Prev buttons and inserting custom link

Hi all,

I am using pagination for some of my longer posts and I want to be able to insert a custom link into the final pages next button.

So let’s say there are 5 pages, on the fifth page normally it wouldn’t say next again because it is the last page but what i want to do is when on the final page of have the next button appear again but link to another url on the site. Similar to a next post button but i want the same next page buttons to just do it for me.
So next page goes to page 1 – 5 and then on the fifth is goes to different url.

How would I do this? I use the standard <!–nextpage–>