When registering a new key to the digital lock, the registration data of the current key will be overwritten by that of the new one which makes it necessary to gather every needed key for registration. 
However, it is possible to register additional key without losing the current registration data by going to 「Digital lock settings」→「Additional registration」 and enable the function.

【How to set up】

Please go to My Entrance home screen's top left corner and select 「≡Menu」 → 「Digital lock setting」.