Based in this issue I need to export in XMI format and use DKPro Core to convert to Brat format:
https://github.com/webanno/webanno/issues/328
I tried this code but did not have success
public void convert() throws Exception {
    SimplePipeline.runPipeline(CollectionReaderFactory
            .createReaderDescription(XmiReader.class, XmiReader.PARAM_SOURCE_LOCATION, "/tmp", XmiReader.PARAM_PATTERNS,
                    XmiReader.INCLUDE_PREFIX + "*.xmi"), AnalysisEngineFactory
              .createEngineDescription(BratWriter.class, BratWriter.PARAM_TARGET_LOCATION, "/tmp"));
    }
				
                        
The dialect of the brat format may be different between what the DKPro Core BratWriter produces and what OpenNLP expects - the brat file format is quite flexible.
If you are using the built-in Named Entity layer in WebAnno, then I would propose an alternative route:
OpenNlpNamedEntityRecognizerTrainercomponentThat should avoid the need for the additional conversion step.
Disclosure: I am a WebAnno and DKPro Core developer.
Suggestions that didn't work:
CoNLL 2002in WebAnnoConll02NameSampleStreamsupports only certain languages and named entity types... sigh