Excelなら２行で実装できますが、Libreではそうもいきませんね。

ポイントはIs_xxx_LineValidメソッドを全てTruenにすることらしい。

Sub ksTableBorder()
Dim objSheet As Object
Dim objRange As Object
Dim objTableBorder As New com.sun.star.table.TableBorder
Dim objBorderLineBlack As New com.sun.star.table.BorderLine

objSheet = ThisComponent.CurrentController.ActiveSheet
objRange = ThisComponent.CurrentController.getSelection()

‘黒の単線の設定
With objBorderLineBlack
.Color = RGB(0, 0, 0)
.OuterLineWidth = 20
End With
With objTableBorder
‘
‘表の外枠の設定
.LeftLine = objBorderLineBlack
.RightLine = objBorderLineBlack
.TopLine = objBorderLineBlack
.BottomLine = objBorderLineBlack
.IsLeftLineValid = True
.IsRightLineValid = True
.IsTopLineValid = True
.IsBottomLineValid = True
‘
End With
‘
objRange.TableBorder = objTableBorder
End Sub