Create custom VisualState in xaml and manually set it in CodeBehind

Have you tried VisualStateManager.GoToState

Takes a Control, string with the custom state name and a bool flag for using transitions.

Example Usage From msdn

private void UpdateStates(bool useTransitions)
{
    if (Value >= 0)
    {
        VisualStateManager.GoToState(this, "Positive", useTransitions);
    }
    else
    {
        VisualStateManager.GoToState(this, "Negative", useTransitions);
    }

    if (IsFocused)
    {
        VisualStateManager.GoToState(this, "Focused", useTransitions);
    }
    else
    {
        VisualStateManager.GoToState(this, "Unfocused", useTransitions);
    }
}

---

A slightly more complicated example usage from here

Given this xaml

      <Grid x:Name="LayoutRoot"  Background="LightBlue">
          <VisualStateManager.VisualStateGroups>
              <VisualStateGroup x:Name="SG1">
                  <VisualStateGroup.Transitions>
                      <VisualTransition GeneratedDuration="00:00:01">
                          <VisualTransition.GeneratedEasingFunction>
                              <ElasticEase EasingMode="EaseOut"/>
                          </VisualTransition.GeneratedEasingFunction>
                      </VisualTransition>
                  </VisualStateGroup.Transitions>
                  <VisualState x:Name="SG1Normal"/>
                  <VisualState x:Name="SG1EllipseRight" >
                      <Storyboard>
                            <DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Duration="00:00:00.0010000" Storyboard.TargetName="ellipse" Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[3].(TranslateTransform.X)">
                              <EasingDoubleKeyFrame KeyTime="00:00:00" Value="320"/>
                          </DoubleAnimationUsingKeyFrames>
                      </Storyboard>
                  </VisualState>
              </VisualStateGroup>
          </VisualStateManager.VisualStateGroups>
          <Ellipse x:Name="ellipse" Fill="Red" Stroke="Black" 
                   Height="116" HorizontalAlignment="Left" Margin="50,98,0,0" 
                   VerticalAlignment="Top" Width="235" RenderTransformOrigin="0.5,0.5" >
              <Ellipse.RenderTransform>
                  <TransformGroup>
                      <ScaleTransform/>
                      <SkewTransform/>
                      <RotateTransform/>
                      <TranslateTransform/>
                  </TransformGroup>
              </Ellipse.RenderTransform>
          </Ellipse>
      </Grid>

Changing state can be done like so.

 VisualStateManager.GoToState(this, SG1EllipseRight.Name, true);

Or alternatively

VisualStateManager.GoToState(control, "SG1EllipseRight", true);