I don't think it's possible (for now): http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/#links
But you can define role
to produce HTML class, for example:
http://discuss.asciidoctor.org[Discuss Asciidoctor, role="external"]
will produce:
<a href="http://discuss.asciidoctor.org" class="external">Discuss Asciidoctor</a>
You can also open an issue here to ask for it!