Have you taken a few minutes to look around the Rom Hacking section, there are 2 ways to do so...
Instructions included with both...
The 2 I can think of would be 1: There was an OOOOLLLLLDDD issue of a Rom Hacking Newsletter that Pokecommunity used to have. I was a subscriber and and Issue I read had info on just using writebytetooffset (The command in XSE) to change it, but it is not a permanant change in the game.
What I would recommend you doing is checking out a thread by JPAN, about a Hacked Fire Red Engine, that has a bunch of new 'Special' commands (like in XSE) that lets you do things like that, like change the OW sprite of your trainer.
Go check it out, I think it is in the toolbox.