To migrate "Account Credits" from a previous CRM software you can create a negative product to add to the cart from the POS/Cash Register.
Make sure the product requires a customer and has a GL Code and/or Category that you can track for reporting purposes. Setting the price at -$0.01 allows you to adjust the quantity as needed, using the Qty field in the shopping cart..
As long as the shopping cart is negative when you check out the 'Apply as account credit' payment type will appear.