In the Configuation Section Explorer, you can define an enumerated type and set "Is Flags" to true.
But it took me half a dozen attempts to figure out the syntax for specifying multiple flag values in my xml, and I couldn't find the answer when searching the documentation or discussions. So hopefully this post will help someone else in the same situation.
Separate your values with commas! So for example, an attribute in your xml might look like this: modifierKeys="Ctrl,Shift"
Maybe that's obvious to some people, but I had tried the bitwise-or symbol '|' and a space, but they don't work.