where can i add database items to the $_session?

In File system_load.php on root on line 67, system checks if user is loged in. As system_load.php is part of every file and loads before anything else. 

See code there. 

if(isset($_SESSION['user_id'])):
		$new_user 		= new Users;
		$user_status 	= $new_user->get_user_info($_SESSION['user_id'], 'status');

get_user_info() is a function which takes user id, as first argument and 2nd argument as database table column name of users. 

Let's say you have created a new column in users table called, mytable so you can get its value for loged in user like >>

$mytable= $new_user->get_user_info($_SESSION['user_id'], 'mytable');

Now variable $mytable have value of column mytable for the loged in user. 

But as this system_load.php file loads on all pages, no matter how many times they refresh. So i would recomment you to check isset($_SESSION["your_sessioon"]) and if that is not set then only set the value. 

Example: 

Your two new columns for example, height and weight. Are two new columns in users table. 

if(!isset($_SESSION["user_height"])) {
        $_SESSION["user_height"] = $new_user->get_user_info($_SESSION['user_id'], 'height');
}
	
if(!isset($_SESSION["user_weight"])) {
		$_SESSION["user_weight"] = $new_user->get_user_info($_SESSION['user_id'], 'weight');
}

I hope this is helpful, there is also another table user_meta which can save user_id, field name and its value. You can also see that.