Harish,
I believe modifying the custom fields in Line Modify won't work.
Please refer this link : Error Custom Fields MB_MIGO_BADI
My idea is to transfer using field symbols and modify it in your custom screen and also at the POST_DOCUMENT.
Eg: assign ('(SAPLMIGO)GOITEM') to <FS>.
Also you need to write your code in the POST_DOCUMENT by use of above field reference.
Thanks,
SaiKrishna