Could it help? Sure. Will it help a lot? Probably not.
Most people go the other way around. You can bring a lot more of what you learned in a PCC fellowship training to a Sleep Medicine fellowship than the other way around. A few SM fellowships actually have tracks for people who are not of the pulmonary background that gives them extra training in pulmonary so that they can better understand and diagnose sleep pathologies.
Focus on boosting your PCC application with things like research, away/internal rotations with PCC physicians and getting good LORs, or more ICU experience. Those will go further in making your app stand out. Not to mention the issue of focusing on programs that have their department core in PCC (the division of Sleep Medicine falls under various departments depending on the program) to help with your connections which would limit your application further aside from the fact that you'd be competing mostly with guys who have already done PCC and applying to SM.