Can I add an arbitrary RSS feed to Flipboard?

Actually, it can be done very easily: Just put the name of the feed, or its URL, in the search box. Flipboard will find it. And then, you can select it, and add it.

Actually you CAN load a specific feed to Flipboard. Start by adding Google Reader. Then launch into it, and click on the downward facing Triangle next to the headline "Google Reader." It will then pop up a window showing you your various feeds. Select the one you want, and its name will appear as the new headline and you will only see stories from that feed. Then click the little "+ content" symbol in the upper left, and it'll add just this single RSS feed to your main flipboard page.

You are correct that you cannot add an arbitrary feed, and as @Mike Scott noted, you have to add your entire Google Reader account.

Frustrating because Flipboard is absolutely not a good way for me to read the bulk of my feeds -- Reeder is perfect for them -- but I'd love to add a few feeds that I rarely read to Flipboard, feeds that fit its visual style.

You can add an arbitrary RSS feed by creating a Google Reader account, adding the feed to that, and then linking it with Flipboard.

From the Flipboard tips page:

Discover Great RSS Feeds. You can use the Google Reader service to access RSS and then read and interact with your feeds right in Flipboard. You can also use the Search feature to find any RSS feed by name, website URL, or RSS link.