Thanks for replying. But what happens if I export the same file second time? Will it overwrite or create new file or append the new data? I think Nessus 5.x doesn't have the API support. It is creating the .nessus file in the backend.
The Nessus file is always created based on the currently selected scan & history. As for Nessus 5.xx, you are aware that it was EOLed, right? Plugins are also no longer written to support Nessus 5.xx, so there is a growing body of plugins and require some newer features of the Nessus 6.xx engine.