You can not make change in HTML of user control from Parent Page at desgin time. If you need to add any column in user control gridview at design time then you need to do it directly in user control .ascx file .All the changes you made will be visible in every page where you use this user control.
Only way you can add new columns in user control Gridview is through code behind dynamically. Please follow below link for dynamically adding columns in Gridview
Add Dynamiv Column to Gridview In ASP.Net Using C#[
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