Can I add a Face ID to iPad mini 5th generation?

FaceID requires integrated hardware that has to be built in to the device from the factory. It is not something that can optionally be added later. Similarly, touchID also cannot be added to a FaceID capable device for the same reason. These biometric systems require built in hardware senders. A device either has them, or it doesn’t.

No.

Face ID cannot be added than iPad mini on all version.

Face ID is supported on iPad Pro 11 inch all generations and iPad Pro 12.9 inch 3rd generation and later

Absolutely NOT!

Sorry.