Most of the Growler "magic" is in pods carried on external hardpoints, but you couldn't just upload the EW pods onto a FA-18F Super Hornet and have a Growler.

The cannon in the nose of a regular Hornet is replaced with computer hardware in the Growler. There are also aerodynamic changes to make it a little more stable so it's a better EW platform. There's probably a million other small differences too, enough that you wouldn't try to convert a -19F Super Hornet into a Growler (although the RAAF did think they'd try at one point).