This is not something that is supported out of the box. You can do it, but you need to manipulate the parsed document yourself. You can do so using a TreeProcessor.
What you are looking for is the `numeral` property on the section nodes. This property accepts a string, so you can set it to whatever you want.
Here's an example:
Asciidoctor::Extensions.register do tree_processor do process do |doc| sections = doc.sections sections.numeral = 'i' sections.numeral = 'ii' (sections.slice 2, sections.length).each do |sect| sect.numeral = sect.numeral.to_i - 2 end nil end end end