https://gbatemp.net/threads/release-agb_firm-signature-patcher-gba-rom-converter.390313/
With 3DS Simple GBA Converter, you can convert them to .cia for use with AGB_FIRM.
I'm not sure if you absolutely need to install the patched AGB_FIRM, but I haven't done that step personally (also on rxTools), and GBA games worked fine. If stuff crashes, then I guess try installing the patched AGB_FIRM or something.
You have to install them both on sysnand and emunand. Inject FBI in H&S app on both sysnand and emunand, install on emunand, then I use devmode to run FBI on sysnand and I install them there too.
Then you can run them straight from emunand and they will work.
(I'm not sure if rxmode sysnand would work too to install the .cias, but I used devmode and it worked, didn't seem to give me any problems, so...)