I am not sure if this is possible. Below is the enum I have always seen of the formats available to save as. (from SaveFormat, in AcApDocument)
kUnknown -1 Invalid format
kR12_dxf 1 AutoCAD R12/LT2 DXF (*.dxf)
kR13_dwg 4 AutoCAD R13/LT95 Drawing (*.dwg)
kR13_dxf 5 AutoCAD R13/LT95 DXF (*.dxf)
kR14_dwg 8 AutoCAD R14/LT97 Drawing (*.dwg)
kR14_dxf 9 AutoCAD R14/LT97 DXF (*.dxf)
k2000_dwg 12 AutoCAD 2000 Drawing (*.dwg)
k2000_dxf 13 AutoCAD 2000 DXF (*.dxf)
k2000_Template 14 AutoCAD 2000 Drawing Template File (*.dwt)
k2000_Standard 15 AutoCAD 2000(+) Drawing Standard File (*.dws)
k2000_xml 16 AutoCAD 2000(+) DesignXML v1.0 File (*.xml)
k2004_dwg 24 AutoCAD 2004 Drawing (*.dwg)
k2004_dxf 25 AutoCAD 2004 DXF (*.dxf)
k2004_Template 26 AutoCAD 2004 Template File (*.dwt)
k2004_Standard 27 AutoCAD 2004 Drawing Standard File (*.dws)
kNative k2004_dwg Current DWG version
kR15_dwg k2000_dwg Alias for k2000_dwg
kR15_dxf k2000_dxf Alias for k2000_dxf
kR15_Template k2000_Template Alias for k2000_Template
kR15_Standard k2000_Standard Alias for k2000_Standard
kR15_xml k2000_xml Alias for k2000_xml
kR18_dwg k2004_dwg Alias for k2004_dwg
kR18_dxf k2004_dxf Alias for k2004_dxf
kR18_Template k2004_Template Alias for k2004_Template
kR18_Standard k2004_Standard Alias for k2004_Standard
