Using php session inside wordpress?

I installed plugin for paypal payement, and it uses wordpress session.

Login page is outside wordpress in codeignitier, i want to use session what is defined outside wp inside wordpress. I can call it like
$_SESSION['key'] .

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Plugin has some global variables like $current_user what is array with all user data.

Can i set session/cookie outside to be like is defined inside wordpress, or i can switch wordpress to use sessions defined outside ?

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