I've been investigating this exact problem lately.
One thing I've found is this NPM package made by Mapbox which should be able to convert geoJSON style to mapnik's XML. But it doesn't seem to work.
Another package stating the same capabalities is this one. It generates some sort of XML. But when I try to use it on our tile server mapnik is unable to render tiles. It fails either on missing font-face
properties, missing marker files or invalid filter expressions.
This probably won't help a lot but might point you somewhere at least.
Also, this question is quite old. It would be awesome if you could share how did you solve it. (If you did.)