Hi Janardhan,
Displaying the Credit card Charges in the same field as that of the amount will not be a feasible option, as the value in the amount field will be the one used for payment posting.
Instead you can have a separate custom field in the cash desk screen under the Credit card tab (Screen 1130) by enhancing the structure RFKH0\RFKH1\RFKH2.
Then you can use the event 6005 to calculate the credit card charges and show that in the cash desk screen.
Moreover, can you please elaborate on what you are trying to do for Point 2.
Thanks,
Amlan