1. You need to have all address that need to backup private-key

That will return your first address of the main account
smartcash-cli getaccountaddress ""

That will return list of all your addresses associated with your main account.
smartcash-cli getaddressesbyaccount ""

2.How to get private keys out of 1 smartcash address ?

smartcash-cli dumpprivatekey 1ofYourAddresses

If your wallet is password protected and locked. Unlock your wallet first by typing “smartcash-cli walletpassphrase "your password or passphrase" 120” (120 is how long your wallet unlocked):