Quote from Yejix Zabiru:
How do I combine the SPRINGBALL with HI-JUMP BOOTS? Do I just add 2 to 1000 in the VALUE box?
You'll need a debugger for this. There's a version of SNES9X that has one built into it, so once you get that what you'll want to do is set a breakpoint trigger at 7E09A2 (code that checks acquired items). When it breaks you'll have to decompress the code to see what it's checking for, and if it's checking for springball then all you have to do is change the bit # to that of the hi-jump boots (the bit # should be the same as the value you see for the item in SMILE, and iirc there should only be 4 bit checks for springball).
I recently did something similar when I changed the functionality of the suits for my hack, and it can take a while to find all the right bit checks to change.