I want to extend lambCI cloudformation template but am finding myself lost in barley human-readable json/yml it provides.
Is it possible to reverse-transform CF template to something else(sam, serverless...), edit it and then transform back?
That template is not really complicated. Having it as a SAM-template wouldn't make it more readable at all, because the ressources it defines either can't be defined using SAM or wouldn't make it significantly shorter.
As far as I know there are also no tools available which would do conversion to SAM/Serverless automatically.
I always find such templates way more readable in YAML, than in JSON, so what you could do is to convert it from JSON to YAML using aws-cfn-template-flip, edit it and convert it back afterwards (or even use it as YAML).