How to Populate a List<string> in XAML?

Here's another easy XAML question for you guys:

I can populate a 'complex' list okay in XAML like:

<local:People x:Key="family">
    <local:Person Name="The Babe" Age="45"/>
    <local:Person Name="Greggles" Age="41"/>           
    <local:Person Name="Elmo" Age=10"/>
</local:People>

But in the case of:

public class FileNames : List<string> { }

...how are the strings added?

<local:FileNames x:Key="fileNames">
        ???
</local:FileNames>

BTW You may recongnise the example, adapted from "Programming WPF" by Chris Sells.

Thanks for your help!