Not sure what you mean by "conversion to mapnik xml/json", imposm3 is a tool to import osm data into postgresql, mapnik create tiles from postgresql data using an xml to describe what to display and how. You can specify the way imposm import data, and you can specify the way mapnik read and display data, so there is not really a conversion here.
I'm guessing you want a style that uses imposm default import. You can find an old exemple using toner light here (not sure if this still works): https://github.com/jplusplus/mapnik-styles or as user30184 said osm-bright: https://github.com/mapbox/osm-bright/blob/master/imposm-mapping.py
Usually, imposm is more commonly used to import data for analysis, and osm2pgsql is used when you want a classic tile server, which explain why there is not much out-of-the-box solutions. There shouldn't really be a need to use imposm if osm2pgsql does works, if you just need to tweak a little the display, you can just either modify the style or just add another layer with what you want to display. At least most people just use osm2pgsql for this reason.
I suppose you try to have fast rendering, but I'm not sure if the pre-processing of imposm with simplified geometry does a big difference on a classical tile server, mapnik is usually pretty fast and there should be more to gain elsewhere with cache and optimizing the style for exemple.
If you want to make your own style or personnalise one, you can look into mapbox studio, or the not maintained anymore tilemill (which I still use personally and works well), or magnacarto which is made by omniscale, the ones that makes imposm.