A good question indeed! While we still don't have NPM node_modules as a first-class citizen in NativeScript CLI, we have it planned for the nearest future
For the time being, the only option is manually copying the js files of the third-party NodJS library and its dependencies to the tns_modules folder of your application and fixing their paths. This is tedious work for the maintenance is really costly, but at least it is an approach...
We are working on getting the real NPM support though...