You can create a bank account for an existing customer. After creating the bank account, you can then create a source (mandate request) for that bank account and customer.
If you want to manually enter a customer's bank account details, you can do so in the dashboard.
1. Select Customers from your dashboard menu.
2. Search for and select the customer.
3. Click Create > bank account in the top right.
4. Enter the required details.
5. Click Save to create the bank account.
You can then proceed to create a source for this customer.