The GA add-on is protected from being triggered by Google Apps Scripting. So you have a couple options.
1) Use GAS and formulas to filter and manipulate the data imported by the GA add-on.
2) Use GAS to query GA and insert the data into the spreadsheet. Basically, you could write your own version of the GA add-on. There is a learning curve but it's doable.
There is a lot of help on Google forums, Stack Overflow and YouTube. I also found a great tutorial on Lynda.com. Since you call yourself a novice, I assume you have a grasp of GAS. You just need the time to work at it.