The Internal to Piece/Piece to Internal menu allows you to convert internals to pieces and vice versa and includes the following options:
Convert Closed Internal to Piece
Convert Internal to Piece with Internals
Convert Piece to Internal
Convert Piece to Internal with Internals
Pieces to Rectangle Holes
Copy Closed Internal as Piece
Copy Closed Internal as Piece with Internals
Copy Piece as Closed Internal
Copy Piece as Closed Internal with Internals