Javascript cookies and crossing domains

I only have one secure domain

[AWildman]

I am only allowed to have one secure domain name: secure.domainname.com.

I need to use this secure domain to read a cookie with a different domain name on the same server: www.somesite.com

Will adding a subdomain with the name of the domain of the site that is on the same server work?:

secure.domainname.somesite.com

so that when setting the domain I can simply use domain = .somesite.com?

I'm still a novice at the whole cookies thing, so please be gentle.

Oh, and if the above is possible, what is involved in setting up a subdomain?

[DrDoc]

You can access cookies across domains, as long as the cookie has a compact P3P policy attached to it.