- Create an npm (hosted) repository to use as your private registry.
I believe you have already done this. - Create an npm (proxy)
repository pointing to your external repository of choice
(https://registry.yarnpkg.com or https://registry.npmjs.org/). - Create an npm (group) with your private registry on first position and the proxy registry on second position.
- Point yarn to your group repository:
yarn config set registry http://nexus.local/repository/npm-group/. If required by your Nexus configuration, authenticate yarn:yarn login.
Also be wary of yarn using any npm config you might have over it’s own: https://github.com/yarnpkg/yarn/issues/4862
Update for Yarn v2+:
Yarn v2+ uses a different configuration key for updating the NPM registry, called npmRegistryServer (see their Migration page for details):
yarn config set npmRegistryServer http://nexus.local/repository/npm-group/