In this section we'll go over how one can insert assembly hacks using EA Buildfiles.
You're going to need the ASM beginner pack (where is it?). Included is AssembleARM.bat and (a very small subset of) DevkitARM (a common development kit for the GBA).
Assuming your .asm file works properly, all you should have to do is drag and drop your file onto AssembleARM.bat, and it should output a .dmp file containing your assembled asm. All you need to do is to insert it: ALIGN 4 (for good measure) and #incbin it, and you should be good to go. You're also going to want to put a label to be able to locate your routine(s) from other parts of your buildfile.
If this is something you ASMC (call through events), don't forget to add 1 to your routine pointer/label. That way, the game knows it's reffering to Thumb ASM.
Otherwise, you're probably going to want a hook. Check out Hack Installation.txt for different ways to hook your ASM. If your hook involves a BL, you need to insert your ASM within "bl range" (for example, in FE8U, in the free space range starting at 0x1C2270).