I found a solution:
<Button Content="v" Grid.Column="1">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Tap">
<eim:ControlStoryboardAction ControlStoryboardOption="Play">
<eim:ControlStoryboardAction.Storyboard>
<Storyboard TargetProperty="(Border.Height)" TargetName="Slider">
<DoubleAnimation From="0" To="60" Duration="0:0:2" />
</Storyboard>
</eim:ControlStoryboardAction.Storyboard>
</eim:ControlStoryboardAction>
</i:EventTrigger>
</i:Interaction.Triggers>
</Button>
Apparently you can define the storyboard inside the ControlStoryboardAction, which was exactly what I needed.