The content for these packs is agreed upon between the creator and Paradox Interactive, then implemented by us. They are very unlikely to be expanded after release, but the store page should contain a description and screenshots to help you determine if it contains something you would enjoy.
For additional Japanese buildings, Beer Fiend is correct that the Steam Workshop is your best option. Ryuichi Kaminogi, the creator of the Modern Japan pack, has a lot of Japanese buildings on their workshop available for free to anyone who has the game on Steam.
steamcommunity.com