Intentional Mama is a lovely website about teaching French to kids. http://intentionalmama.com/home/making-your-childrens-books-bilingual this post was about writing in books to make them bilingual.
I have to admit that usually I have enough English books and Japanese books. (We either buy them or get them from the library.)
However, occasionally I have really wanted to read a book in English, but I don’t have the English version. And I don’t really want to buy it.
What I have done is borrow the Japanese picture book (always a picture book! I am not a professional translator 🙂 ) from the library, and then translated it into English. I write the English into a notebook, which I then read to my son during cuddle time.
It is not ideal, of course. But it works.
One major problem is that it takes a really long time for me. But it is good Japanese practice. But I am not really motivated to study Japanese through picture books. (I use other resources.)
I have never actually written in a book itself. I have never really had to. But if there were no English books to be found? (This is the reality in Japan for virtually every foreign language except English.) Then, yes, I would definitely write in my own Japanese books. Definitely!