Using Feeds to import into existing nodes?

For a site I'm building, I need to do some exporting/importing. Nodes are added to the site normally through the UI and some of the fields are filled in at that time. Once all the nodes for that month are added, the data is exported to a file, likely XML or CSV. So I'll end up with something like this:

NID,TITLE,FILENAME,SCORE1,SCORE2,SCORE3,PLACE
42,"a pretty picture","a/nature/xyz.jpg",0,0,0,""

An external judging program will fill in the scoring information so now the file looks like this:

NID,TITLE,FILENAME,SCORE1,SCORE2,SCORE3,PLACE
42,"a pretty picture","a/nature/xyz.jpg",25,30,28,"HM"

I then need to go back to the existing nodes and take the new data from the file and fill in the appropriate fields. This will be done once each month, after the judging, and only be run once per node.

When I asked on IRC, someone said this could be done with Feeds. I've been looking at the documentation and all I'm finding is making new nodes from import files. I haven't found anything that will look at the NID in the file and go find that node and import the data from that line.

Before I spend more time on this, could you tell me if this is something Feeds can do? If so, I'd appreciate any general pointers. I'm willing to RTFM and do digging on my own but I'd like to avoid chasing wild geese if I can. :)

Thanks,

Michelle