Using Other Site's Session Data In Yii Site

Hello, I have one (non-Yii) mobile site. In that site the session id is traveling in the urls.
Now, I want to add another site to the old big portal. The new site is in subdomain and have to use the session of the old site, but now I have to save the session id in cookie.

How can I set up my Yii site to grab my session id, save it to cookie and use this session?
I absulotely can't figure it out.

Edit: in the Yii site, I don't need login or authenticatin. This stuff is done in the main site.