Hi Satish,
The enhancement you are mentioning cannot be through conditions. Your SAP screen needs to be maintained with those extra check boxes. I would not suggest to make those changes even if you copy the standard PO program into a custom one.
If I understand your requirement correctly, I would create a condition type with the calculation type D or E. I have never used this, but you can evaluate the usage of these calculation type to see whether they meet your requirement or not. See the screen shots below:
Definition of condition type: (Here D is for gross weight and E for Net weight)
Condition record where the price is as per the weight:
With this, in the PO, the price will be calculated by the weight.
Try to figure out creating a condition type for net weight and gross weight for your requirement.
Let us know if you need further details.
Regards,
Amit