In-app purchase from Japanese app
Yes that is possible. You can buy gift cards from other country iTunes Stores. 

One quick search and I found https://japancodesupply.com/, where you can buy Japanese iTunes Gift cards. When you order it takes approximately 1 minute to send you an email with your code.

There are many other websites that offer iTunes Gift cards from other countries. So do a quick search, check the credibility of the website and buy a giftcard.

I'm not affiliated with japancodesupply.com