Hi Satheesh,
If you are not entered Mfg Serial Number at the time f GRPO making, you can update it after GRPO also. For same You may open GRPO and open Serial Number screen then you will find MG Serial Number column with golden arrow, just click on it and enter your missing number.
Also you can go to Inverntory->Item Management->Item Serial Numbers->Serial Number Management. It will come "Serial Number Management" screen then select item, system will show you all serial numbers and you can fill Mfg Serial Number in "Mfg Serial No." Column.
And if you want to do it through DTW then goto "Data Transfer Workbench\Templates\Templates\Purchasing\Goods Receipt PO" there is template name as "SRNT - SerialNumbers.xlt", you can fill with GRPO number and line number with your missing information and update it.
Remember if you want to update GRPO then go to above mentioned path in DTW Template folder or else go concern folder and you will find same template for update it.
Hope this will help you to resolve your issue.
Regards,
Sachin D