It is generally considered bad form to leave the name of the game an acronym. I might know from context but not everybody will. Others might think Super Monkey Ball, Super Meat Boy or something else.
Simple way is to find the sprites that are the hammers and change them. Though in this case you will probably also the change the hammers from later levels Bowser which might not be what you want. Still
http://datacrystal.romhacking.net/wiki/Super_Mario_Bros.:ROM_map should get you started here.
Beyond that you can change what either Bowser throws and sort that or change what the hammer bros throw and point it at another thing. This might be easier said than done.
This is just a graphical thing though. To change patterns of things thrown you will need to find where those are kept.
http://datacrystal.romhacking.net/wiki/Super_Mario_Bros.:RAM_map mentions some of where it is in RAM so you can debug it from there, though Super Mario Brothers is also one of the few games to get a commented disassembly so you also have that.