Now I am not 100% sure on this but this solution makes sense to me...
You should be able to pass the variables via GET to the new domain your redirecting too.
Lets say you have variables declared in your session a,b,c
you should be able to redirect to the new domain in this fashion and then the other server (sercure site) would have access to them.
<a href=securesite.com/somepage.php?a=$a&b=$b&c=$c>
I've never actually done this but I'm currently working on a project that will require somehting to this extent, and this was how I was going to try and acheive it.
Let me know if it works.
Thanks