Should this instead look for a ConllNerSpanCollection?  If a previous annotator puts in a ConllNerSpanList this step won't find it.  Also, perhaps we don't necessarily want to create "ConllNer" version of SpanCollection, SpanList and SpanBuffer.  Perhaps instead something like "TokenSpanCollection with ConllNerSpans", with the trait just being a marker?  I'm open to alliterative proposals.