Can I install a NPM package from javascript running in Node.js?

It's possible to use npm programmatically, as stated in the documentation. Here's the code example they provide:

var npm = require("npm");
npm.load(myConfigObject, function (er) {
  if (er) return handlError(er);
  npm.commands.install(["some", "args"], function (er, data) {
    if (er) return commandFailed(er)
    // command succeeded, and data might have some info
  });
  npm.on("log", function (message) { .... })
});

Since npm is in the node_modules folder, you can use require('npm') like any other module. In your case you specifically will be looking at the npm.commands.install() method:

If you need to look in the source then it's also on GitHub. Here's a complete working example of the code, where there are no options to pass:

var npm = require("npm");
npm.load(function (err) {
  // catch errors
  npm.commands.install(["ffi"], function (er, data) {
    // log the error or data
  });
  npm.on("log", function (message) {
    // log the progress of the installation
    console.log(message);
  });
});

Note that the first argument to the install function is an array. Each element of the array is a module that npm will attempt to install.

yes. you can use child_process to execute a system command

var exec = require('child_process').exec,
    child;

 child = exec('npm install ffi',
 function (error, stdout, stderr) {
     console.log('stdout: ' + stdout);
     console.log('stderr: ' + stderr);
     if (error !== null) {
          console.log('exec error: ' + error);
     }
 });

it can actually be a bit easy

var exec = require('child_process').exec;
child = exec('npm install ffi').stderr.pipe(process.stderr);