You can definitely try to do this.
As an FP resident, you can get some credit for IM. If your program has an IM residency and if you do a rotation where you are treated exactly like an IM resident, you can get 1:1 credit for that. Otherwise, you can get a maximum of 3 months of credit no matter what you do. Advanced credit is at the discretion of the IM PD -- they must apply to the ABIM for it for you.
One of the easiest ways to do so is to apply via ERAS for PGY-1 spots. Obviously, it is way too late to do so for this year (NRMP deadline is right now, 9PM tonight!). If you're only going to get 3 months of credit, most PD's will have you repeat the whole year.
At this point, your best option is to contact PD's directly. You'll be looking for a pGY1 spot.