Users Enter Own Amount?

You mean their bank account number ?
HikaShop, as any other ecommerce solution, doesn't process the payments directly but always goes through a payment gateway. So if you ask me if it is possible with HikaShop, I can say yes, but you're not asking the correct question to the correct person. You need to see which payment gateways offer that payment method for the customers and in these payment methods, which have a plugin available for the ecommerce solution that you want to use.

PayPal, in some countries, allows the user to give their bank account information instead of credit card in order to pay with their PayPal account and that their bank account gets debited for the payment. There is however a delay of several days for such "e-check" payments (as PayPal calls it) so orders wouldn't be validated directly. And as you probably know, there is already a payment plugin for PayPal for HikaShop. So that is one potential solution.
Some other payment gateways probably offer such similar service but you would need to ask them about the details as I don't know what each payment gateway is capable of.