You can combine multiple Nessus reports into a single file.
#Use of:
1-) Download all scans to be merged from Nessus with the .nessus extension.
2-) Collect all .nessus extension files in a single directory.
3-) Then, when you run the tool as follows, a file named "combined_results.nessus" will be created in the same directory.
python3 nessus-merge.py
4-) When you import the newly created "combined_results.nessus" file into Nessus, you will see that all scans have been combined.