This makes the Bar style inherits from Foo
<ResourceDictionary x:Class="Baltz.Styles.Bar"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008">
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="Foo.xaml"/>
</ResourceDictionary.MergedDictionaries>
<Style x:Key="aKey" BasedOn="{StaticResource StyleInFoo}" TargetType="{x:Type aType}">
<Setter Property="aTypeProperty" Value="aTypePropertyvalue"/>
</Style>
</ResourceDictionary>
As always, feel free to comment, or ask.