How do you make a non blowback airsoft gun a blowback airsoft gun?

You can't, really.

An automatic Airsoft gun runs on two types of power sources. Electricity and Gas.

Basic Electric Function:

Basically a standard battery will power a simple 2 wire transfer with a fuse that varies. the power will be connected to a motor when the trigger is squeezed, connecting the power source to the motor. there is a plate on most models that controls wether the motor makes one revolution or multiple ones. At the end of the motor is a gear known as the pinion gear. this will spin 3 gears named the spur sector and bevel gear. these gears will pull back teeth on a piston held forward by a spring which is held in place by a spring guide. once the gears pull the piston all the way it releases and shoots into the cylinder compressing air that shoots out of the air nozzle, into the hop up to propel the BB forward.

I am not a specialist by any means in gas guns, but by all means, try this:

The hammer hits a valve releasing a tiny bit of gas. everytime a new BB is chambered, depending on the model but for this I will just refer to a gas blowback M9, the slide goes back. pull the trigger again, release the gas propel the BB forward and chamber a new round. full auto simply allows the slide to move back and forth freely until locked into place by the trigger being disengaged.