I also need to be able to add rules programmatically preferably by rest api. I think its one of the must requirement for big companies to be able to use camunda as the vendor for their decision and process engine.
https://docs.camunda.org/manual/7.12/user-guide/model-api/dmn-model-api/create-a-model/
In order to update an existing DMN you have to retrieve it via the RepositoryService and the change the DmnMdelInstance.
https://docs.camunda.org/manual/7.12/user-guide/model-api/dmn-model-api/repository-service/
Here is an example to programatically generate dmn decision table and deploy it to a running process engine via REST endpoint. https://github.com/roshinirevindranathan/camunda-dynamic-dmn
This is a spring boot application with an endpoint to generate and deploy the dmn decision table. More information can be found in the Readme file.