Is it possible to add another DISC. TAKEN field for an invoice e.g two DISC. TAKEN fields for an invoice in the applied subtab of customer payment record? If it is possible then can we assign it to GL account of our choice?

Not Possible with Native NetSuite

is there any possibility apart from Native Netsuite , i mean anything else that i could possibly look at for this ?

I mean, you could always write a custom suitelet to replace the behavior, but $$$$$