why do you want go to a 2-speed in the first place?
making a custom gear box would be a huge nightmare. best bet would be to see if you can modify a 2-speed box out of another car to fit the Firestorm. dont know much about your truck but i took a look at the HPI website and it looks to be laid out very similar to the Jato. maybe you can do something with the Jato 2-speed trans. still would take a lot to try and get this to work also.
just remember that anything can be done, just depends on how much you want to spend and how soon you want it done