Adding a custom field to Planning layout in CJ40

Hi Experts,

I have the following requirement.

Need to add a Custom field to the Planning Activity Input screen in CJ40 transaction.Could anyone please guide me with the steps  to achieve this requirement.The Planning layout and the profile assigned to this screen is 1-702-IM and SAPIM2 respectively.I tried creating a custom layout by copying the standard layout but I am struck up in creating a new column for the custom field.

Thanks and Regards,

Rohini Shankar