Can you add platform specific code in PhoneGap?

yes you can add as much platform specific code as you want. Phonegap is just for the interaction of the platform specific code and the javascript. Its the plugin mechanism that makes Phonegap what it is.
Even if you have a native app, you can add a bit of Phonegap in it by having a page somewhere which is basically a webview and is manipulated by js. Phonegap is what launches and manages the interactions of the js and the native part of the app.