The custom.php file
In order to add a new module position, edit custom.php file. The process to create that file is explained here.
Duplicate an existing module position
Now, let's say you want a new module position below grid-top2. In this example the new position will be named "custom". First, find the grid-top2 position code in custom.php.
Copy that code and paste below.
As last step, set the "custom" position name by replacing "grid-top2" mentions, including the id.
Selecting the position
Please note when assigning a module to the new position, the name won't be listed, however you can type it and press enter key.