You can do this by pulling the object collection out of the existing pattern, adding the new features to the collection, then putting it back in the pattern definition. The code below should work if you change the patternName and featureName to what you are using - with the option to hand it a Document that you already have, just use that in place of partDoc.
Dim patternName As String = "Bracket Pattern"
Dim featureName As String = "New Feature"
Dim partDoc As PartDocument = ThisDoc.Document
Dim partDef As PartComponentDefinition = partDoc.ComponentDefinition
Dim patternDef As RectangularPatternFeatureDefinition = partDef.Features.RectangularPatternFeatures(patternName).Definition
Dim patternFeatures As ObjectCollection = patternDef.ParentFeatures
Dim newFeature As PartFeature = partDef.Features(featureName)
patternFeatures.Add(newFeature)
patternDef.ParentFeatures = patternFeatures
.
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