Selection.PasteAsNestedTable (Word)

Pastes a cell or group of cells as a nested table into the selection.

You can use PasteAsNestedTable only if the Clipboard contains a cell or group of cells and the selected range is a cell or group of cells in the current document.

ActiveDocument.Tables(1).Rows(1).Cells(3).Range _ 
 .PasteAsNestedTable