yes you can
this guy(Alpha) did it already [ http://projectpokemon.org/forums/showthread.php?9766-HGSS-Hack-Pokemon-Special-DS-(PSDS) ]
but he's not remaking pokemon yellow
you should change it so that:
If you choose Boy at the start you will play as Zekrom.
If you choose the girl you will play as Reshiram.
I read somewhere that the people who designed the pokemons made reshiram ladylike.
i don't think the part of the safron battle is posible to make, cuz they were battleing in midair on saur's vine
oh and i just found this: http://www.serebiiforums.com/showthread.php?t=29049
maybe you can use those sprites for the hack