You can add PS's any time you want. You would be free to apply to IM programs now, and then add FP programs in the future with a new PS. Or, the other way around. Or, you could apply to both IM and FP programs simultaneously. Just be careful not to send the wrong PS.
If you want to apply to both now, and only one PS is written, you can do so and simply not attach a PS to those programs in the other specialty.