If you don't mind some code behind, you can achieve this behaviour by reacting to the DragDelta event of the splitter, and adjusting the width of the first column, by removing the Star width, and fixing the width according to the drag delta.
Xaml :
<Grid Name="GridName">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Border Grid.Column="0" Background="Red" />
<GridSplitter Name="SplitterName" Grid.Column="1" ResizeDirection="Columns" Width="3" Height="Auto" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
<Border Grid.Column="2" Background="Green" />
</Grid>
Code behind
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
SplitterName.DragDelta += SplitterNameDragDelta;
}
private void SplitterNameDragDelta(object sender, System.Windows.Controls.Primitives.DragDeltaEventArgs e)
{
GridName.ColumnDefinitions[0].Width = new GridLength(GridName.ColumnDefinitions[0].ActualWidth + e.HorizontalChange);
}
}