We don’t have such documentation yet, sorry. You can take a look at the code of one of the existing outputs (called collectors internally in k6) here: https://github.com/loadimpact/k6/tree/master/stats, for example https://github.com/loadimpact/k6/blob/master/stats/json/collector.go
Keep in mind that there are plenty of things we want to refactor in the current output API and its configuration (Refactor the collectors/outputs · Issue #1075 · grafana/k6 · GitHub, Refactor collector registration · Issue #917 · grafana/k6 · GitHub), so there would be some rough edges. And we don’t have a plugin system just yet (it’s WIP - Plugin support · Issue #1353 · grafana/k6 · GitHub), so you’d have to either wait for that or fork k6 and add it to the core (which I can’t promise we’ll merge). Out of curiosity, what sort of output do you want to add?