If you wish to export package heights, you should use IDF. The 2D DXF model offers no way to transfer a 'z' value for a polygon. With IDF, you can only transfer a single height and place bound per package. If your symbol is made up of multiple place bounds, Allegro combines them and outputs the largest height value. The newer IDX standard supports a symbol that contains multiple place height bounds but you will need to check if the consuming tool supports IDX.