Hi there. Believe it or not, the feature of merging machine result files (.MRF) is on our list to add, it's just not near the top.
You can keep a "Master" list however and have NEWT Pro either Discovery or Scan only new machines it finds. This is actually easier than merging 2 files together. You can then save the new .MRF over the old one or as a new version.
Uncheck the option "Clear all machines in all tabs" under the Scan/Discovery panel. This will keep your previously scanned list and only add new machines.
There is currently a small bug in this in feature. It will still warn you that the list will be cleared, but it will not. This will be fixed in the next maintenance release of NEWT Pro.