こんな感じ
Workbooks("B.xls").Worksheets("sheet2").Range("A2")=Application.WorksheetFunction.Sum(Workbooks("A.xls").Worksheets("sheet1").Range("B2:B3"))
こんな感じ
Workbooks("B.xls").Worksheets("sheet2").Range("A2")=Application.WorksheetFunction.Sum(Workbooks("A.xls").Worksheets("sheet1").Range("B2:B3"))