There is no Python command to add frames to a scene. So you have to use a little skullduggery. Use the LoadLibraryFigure command to load a cut down PZ3 file.

Tested it to increase or decrease total frame numbers. Worked both ways without Poser raising its "do you want to delete frames" message.

You could even embed the PZ3 into your script, have the numFrames as a variable and get the script to write the file prior to executing LoadLibraryFigure. Yes I know its not a figure but don't tell Poser that :)