You typically can't use the android.nfc.* packages for such a scenario. Android's NFC API is intended for NFC interfaces and not for RFID readers. Even if you do have an NFC interface device connected to your Android device through USB, you would need to provide the hardware abstraction layer to connect your NFC interface device to Android's NFC stack and compile that into your Android system.
Depending on how your RFID reader works, the reader's manufacturer might provide an API for Android (unlikely) or will be able to provide you with specifications on how to interface their device (this could be possibe through ANdroid's USB API but it could also be as simple as receiving keyboard inputs).