Sorry, but I still don't know exactly what you need.
avast! doesn't specifically scan .nsf extension - it doesn't consider this extension to be anything special. So, it will not be scanned during a "quick" scan. However, if you select a Thorough scan (or in the Enhanced User Interface, you specify the task to scan all the files), all the files are scanned.
However, if you are asking if avast! scans the content of .nsf files (i.e. what is inside the database, speaking of the database as an archvie (such as ZIP)), then I say I don't know - because I don't know what format use Lotus Notes. I think avast! doesn't know this format, but I can't say it for sure. However, you can simply try yourself - as I described above.