Travel beyond area of my JR regional pass

Hello,
I am planning to Purchase a JR Kansai-Hiroshima Area Pass for my trip. I fly into Osaka and out of Tokyo and plan to visit Hiroshima and others places in between. The pass is perfect for all of my trip except the final journey. On the final day I will travel from Hiroshima to Tokyo. Do I need to purchase a ticket for the entire trip outside of my pass, or can I book part of this trip (as far as Osaka/Kyoto) with the pass and then purchase a seperate ticket from there to Tokyo? If I do the latter, can I simply stay on the train or would I have to disembark?
I can't find info on this clearly from JR or on Klook so hope someone here can help. Thanks in advance.

Just take your pass ro a ticket window or office and pay the difference the pass doesn't cover. Done.

Hiroshima -> Shin-Osaka, use your rail pass.

Shin-Osaka -> Tokyo, purchase this ticket separately, and ahead of time, so that you don't have to disembark at Shin-Osaka.