This post shows you how you can reference another object through it’s external id. This is useful if you do not know the Salesforce ID but know the external ID.
Example: You have an external field on the Account object with an API named myExternal_Id__c. You have an account record named “Burlington Textile of America” and let’s give the external ID a value of “X123”.
The scenario is you want to create a contact record but do not know the account record id but know the external Id.
Check this snippet of apex code to accomplish this.
The same concept is applicable for JSON files