It depends on how complex the program is, how much it uses stuff in the Objective C environment that would have to be mapped to equivalences in the Arduino environment, whether the program will fit in the Arduino's memory space, and your skill level as a programmer. A lot of times, it might be simpler to start over, and recode it from scratch, using the Objective C program as a template, than it would be to do a mechanical translation. Assuming you have little programming experience, it might be a long and painful task (but presumably if you finish, it will teach you a lot about programming).