Indeed you can't do that...
Here's the easiest way to do it:
- Add popularity to your content schema in Tridion and populate it in the CMS when it changes, then republish
Fredhopper's data model is relatively schema-less. Your data defines the schema, and that's why it copes so well with the Tridion schemas. By adding a field to your content, it appears as an attribute in Fredhopper.
The challenge with doing this using the data manager is the following:
To update one field of one item, you need to first load it from Fredhopper, update the field, load it back (easy enough through Data Manager). Fredhopper will not do "partial updates of one item" so you need to update the whole record.
While this is fine and really not that hard to do, what happens if you republish that item from Tridion? You will NOT have the popularity field until you run the data manager again.
So the usual approach we do with this is to have Tridion deploy the XML to a "staging" location, have a service or similar with either a X minute interval or fancy "file system change trigger" that detects this new file, enrich this file with the popularity (or any other attribute you want) and then move it to the Fredhopper "incoming" folder.
Hope this all makes sense, it's been a very long weeek.