You can find good details on this at Collect Small Slices on a Pie Chart.
Say we have a simple DataSet and a simple chart based on this:
Now, in the Designer, go to the chart Chart Series -> CustomAttributes and update:
- CollectedStyle
- CollectedThreshold
- CollectedLegendText
Now all your small slices are grouped together:
Obviously you may have to tweak slightly for your chart/data but this should give you a good start.