The REST API for ReqIF imports is not public, and I am not aware of any intentions of making it public for now. So the Knowledge Center stating that the API can be used, is confusing. It's part of the private REST API, so you could reverse engineer the requests/responses, but it isn't documented and supported, usage at your own risk.

Ways to integrate with DNG that are available:

• RRC Reportable API - creating custom reports to be run from DNG or RPE, read only.
  

• OSLC RM V2 API - the open standard way to programmatically integrate with DOORS Next Generation, there is an elaborate workshop to get you started here OSLC Workshop article. These programs can be from any language that support HTTP REST calls, e.g. Java, C# etc. Hence these are more flexible to run and can read and update DNG data, as part of a web server, or standalone, but have some limitations set by the current OSLC standard, for example module support.
  

• TRS 2.0 - the OSLC Tracked Resource Set open standard REST API to hook into the stream of low level changes done to resources in the DNG application (and other Jazz apps as well); read only.

• and additionally, there is the so called client extension capability, these are Javascript extension that generally added as widgets you can run from the dashboard, and can be used to view and edit DNG data. It's more tailored to DNG, so has better module support, but is limited to run in a browser/javascript environment.