where can i add database items to the $_session?

I would need to add a few more values to the session array, where can I add them?

I have added new fields to the datatable users and would need them to be loaded in the the session array as well.

can you please indicate in which files these are loaded?

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.