Autodesk AutoCAD should possibly be able to import HPG HP plotter graphics files and save them in IGES ASCII CAD format.
Convert HP Graphics Language plotter file to IGES 3D exchange format.
We found 2 software records in the database related to .hpg to .igs file format conversion.
Microsoft Windows software - convert hpg to igs on Windows
Leading 2D and 3D CAD development software from Autodesk
Convert from hpg file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned hpg to igs. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A CAD viewer program with conversion capabilities