You can use RadGrid's ItemCreated or ItemDataBound event to reference the user control by ID and pass it any value you need:
sender, GridItemEventArgs e)
GridEditFormInsertItem && e.Item.IsInEditMode)
GridEditFormInsertItem insertItem = (GridEditFormInsertItem)e.Item;
Control editFormControl = insertItem.FindControl(GridEditFormItem.EditFormUserControlID);
//editFormControl now references your user control
//you can pass it any value you need here
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