If your game data doesn't already have a CUE file, you'll need to create one. A CUE file is a text file that describes the layout of your game data.

Dating sims, Mahjong trainers, and obscure visual novels that define the 32-bit era in Japan. How to Create Your Own CHD Library

By following these steps, you should now have a full, compressed PSX CHD image for your Japanese PSX game. Happy preserving!