I have not done this in years, but when I did this I used the ASCII out for the cell. Look at the cell header for a good mechanical cell and its general format for the elements held in the cell and match our new cell to them. I think that all I had to do was to change the the header. Import the ASCII file into the library.
Mechanical Cell is the cell without any pin but Package cell must have one pin. I don't think it's possible to change your package cell into mechanical cell in LMC.
If you wanna place those package cells on your board, why not simly place it as spare cells?
You can DXF Export and Import in the Cell Editor.
In the Package cell, go to File - Export - DXF, then select the items you require in the mechanical cell.
OK to save the file.
Open a new mechanical cell. It will need a different name, then File - Import - DXF