Add new dependency to a package

I have added a dependency to the `package.json` file of a package that I'm patching, but this would never actually install that package properly, as the patching of the package.json file would happen after install, and by that time all relevant packages would be installed.

If I add `npm install xyz` with path to the package to the `postinstall` script, it would install all packages of the module, but would cause all sorts of errors in my app.

What would be the best way to patch a package adding a dependency to it in the process?