Is this possible to a user to add there account balance in there account manually
Yes, you can add credit transaction for the customer in admin section.
http://docs.abantecart.com/pages/customers/transactions.html
Hi, check also this https://marketplace.abantecart.com/store_credit
allows customers to purchase store credit via normal checkout process.
With store credit purchase customer's account balance will be increased with the amount of credit purchased.