The Spreadsheet report - Debtors will list all the debtor (customer / client) accounts in a spreadsheet. 

The data in this spreadsheet is the same as the Debtor listing - "List postal addresses" report (Reports → Debtors → Listing (Reports ribbon)).

To import the debtor (customer / client) accounts: 

On the Reports ribbon, select Spreadsheet reports → Debtors.

Disabled (inactive) debtor (customer / client) accounts will be included in this spreadsheet report.