You can export by tag like this:
https://pinboard.in/export/format:xml/?tag=foo
> I also have a friend who has bookmarked several pages for me in her
> account. I want to add them all to my pinboard account (there are
> about 10). Is there a way to "copy to mine" without doing each one
> individually? I guess one workaround would be something like the
> "bookmark for:userx" feature that delicious had, but that seems
> counter intuitive to pinboard. Basically, I am looking down the road
> for a way to help her with bookmarking.
There's no way to do this within Pinboard. It's possible someone has
written a plugin or extension for it.