Contacts Journal can add contacts from your iOS/Mac Contacts app. If you need to add contacts from a different source (e.g MS Exchange, Google Contacts), you should first add that source to the Contacts app.
To add a contacts source, go to:
iOS: Settings app -> Passwords & Accounts -> Add Account
Mac: System Preferences -> Internet Accounts -> Add Account
Once contacts from these accounts are synced and available in the Apple Contacts app, these contacts will become available for import inside Contacts Journal.
Default Account
To choose the default account to which you want to add your contacts to:
iOS: Settings app -> Contacts -> Default Account
Mac: Contacts app -> Preferences -> General -> Default Account.