We have tried to record results of nunit2 execution via warnings-ng plugin. Unfortunately we haven't found a ready to use warnings-ng tool for nunit (we are still using nunit2 and not the nunit3).
So we have used Microsoft tool (mssql.exe) with the xsl-convertion taken from https://github.com/jenkinsci/nunit-plugin/tree/master/src/main/resources/hudson/plugins/nunit (and added the encoding "utf-8" parameter to generate junit-files, which can be consumed by xunit-plugin).
The resulting files could be successfully integrated into the jenkins build reports using xunit-plugin:

xunit ( tools: [JUnit(pattern: '.out\\*.Test.junit-result.xml', stopProcessingIfError: true)] )

But we cannot see any issues in the warnings-ng plugin (using reportIssues as follows):
recordIssues(
                    name: 'Unit Tests',
                    enabledForFailure: true,
                    aggregatingResults: false,
                    tools: [junitParser(id: 'unitTests', name: 'UnitTests', pattern: '.out\\*.Test.junit-result.xml', reportEncoding: 'UTF-8')]
                  )