If you have the need to merge / combine a few Nessus scans into a single *.nessus file, you can do so using this simple Python script. Since *.nessus files are basically just XML files with a different extension, what this script does is it finds all the *.nessus files in the current folder, finds all the “ReportHost” XML nodes accumulating them into a single report.nessus file which is then exported to nss_report folder.

Note that scans must be of the same type (same plugins must be used), but they can be from different subnets or different parts of the same subnet.

How to use it? - Put the script and all your *.nessus files into a same folder, run the script, import nss_report/report.nessus into Nessus - and there you have it, all the hosts are in one place..

#! /usr/bin/env python3.2

import xml.etree.ElementTree as etree
import shutil
import os

first = 1
for fileName in os.listdir("."):
  if ".nessus" in fileName:
    print(":: Parsing", fileName)
    if first:
      mainTree = etree.parse(fileName)
      report = mainTree.find('Report')
      first = 0
    else:
      tree = etree.parse(fileName)
      for element in tree.findall('.//ReportHost'):
        report.append(element)
    print(":: => done.")

if "nss_report" in os.listdir("."):
  shutil.rmtree("nss_report")

os.mkdir("nss_report")
mainTree.write("nss_report/report.nessus", encoding="utf-8", xml_declaration=True)

If you have any questions, just drop a comment bellow..

EDIT: mastahyeti made some improvements to this script, you can get it at his GitHub