There is currently no way to fully copy a table complete with layout and
formatting nested into a cell of another table.

One can create a new table in that cell and copy the original table into the new
table, but this only copy the content, not the layout / formatting of the table
(merged cell, borders, etc.)

One can create a frame inside the cell and copy the original table into that
frame. But since it is a frame, there are some limitation to table behavior,
e.g. it cannot span multiple pages.