FrontendUser::setSessionData() 

\nn\t3::FrontendUser()->setSessionData($key = NULL, $val = NULL, $merge = true); 

Set session data for FE user

// Merge session data for `shop` with new data (existing keys in `shop` are not deleted)
\nn\t3::FrontendUser()->setSessionData('store', ['a'=>1]);

// Overwrite session data for `shop` (`a` from the example above is deleted)
\nn\t3::FrontendUser()->setSessionData('store', ['b'=>1], false);
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| @return mixed

Source Code 

public function setSessionData( $key = null, $val = null, $merge = true )
{
	$session = $this->getSession();
	$sessionData = $merge ? $this->getSessionData( $key ) : [];
	if (is_array($val)) {
		ArrayUtility::mergeRecursiveWithOverrule( $sessionData, $val );
	} else {
		$sessionData = $val;
	}
	$session->set( $key, $sessionData );
	$this->getFrontendUser()->storeSessionData();
	return $sessionData;
}
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