After installing hakchi 2.20 - https://github.com/ClusterM/hakchi2/releases/tag/2.20

Go here: https://github.com/ClusterM/retroarch-clover/releases
Follow instructions and just download retroarch_with_cores.zip, drag/drop in hakchi 2.20 window (this is with the snes classic connected and powered on) and it'll run you through the instructions and then ask which emulator cores to install. These will say which console they're for and you can just pick/choose.

After that, just add games from the menu as normal, select the .gba file of your choice and then add --retroarch to the end of the command line in the command line box on the right side of the menu (you can add --retroarch to any snes game that doesn't play well on the stock snes classic as well, this just forces it to use the retroarch core instead of the classic's emulator) and then hit the synchronize button and your gba game should now show up in the menu like any other, complete with whatever button reset combo you used to go back to menu and even save states.