Adding basic support for Node Reference fields to the Feeds module is
pretty straightforward (if you don't worry too much about the advanced
features of nodereference
).
In fact, it's almost a direct copy of the existing support for basic CCK field
types. Just check for a different field type (line 19) and lookup the nid
based on the GUID from the feed (line 51).
I've used a CSV file with a quoted, comma-separated field containing GUIDs of my target nodes and explode the value using Feeds Tamper.
To "install" this code, just drop nodereference.inc
into the feeds/mappers/
directory and clear your Drupal cache.