update user object in session

[eluser]ChrisHJ[/eluser]
Hi,
could anybody help me with the following:

When a user is loggin in im storing the userobject in a session.

Code:

$this->session->set_userdata('userObject', $userObject)

In the userObject I have an array like this

Code:

$userObject->array

How can I update the array without having to destroy the current session and setting a new one ?

/Christian