Adding a $visitor['myvar'] = 6 in guest sessions?

[Marcus]

From what I guess, sessions are stored in XenForos Database in xf_session, so if I put a variable into the $visitor array, I could track that value during the visit of my guest user.

But how can I store a value into $visitor? In XenForo_Visitor the only write function I see is ::setup($userId, $option). Do you have a clue, where I could start? Currently I work with cookies, but because of security I have to create sessionIds anyway and I don't want to reinvent the wheel.
Adding a $visitor['myvar'] = 6 in guest sessions?

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