Google People stores certain properties (and let's say major apart from basics as person's names or occupation) of a person as JSON string (e.g. Phone numbers, Addresses, Emails or Chats). Such columns are not human-readable (at least not comfortably) as well as editing in Excel lacks the user-friendliness as you can imagine because you have to edit JSON string instead of single value. This means also such data is not filterable and you cannot apply other standard Excel features which shall be the aim of Excel Add-In.

My suggestion is to parse JSON string values as separate columns in Excel and allowing their modification and update back to Google People database. In particular the JSON string values shall be ordered based on their key so the values of the same key appear in the same column (e.g. Home phone number). It should be also possible to add new key to each property (e.g. new relationship type and value, new date type and value etc.).