How to record a previous fall

AFAIK, Apple Health app doesn’t have a means to add undetected falls into the health data.

You can maintain that list separately.

You can send your suggestion to Apple, too: Feedback - Health - Apple

This might help with your own logging, “borrowing” some other place for the logging: Can I log data in apple health (e.g. bloo… - Apple Community

Record it where?

That's not something you can log like extra exercise minutes.