user28017: I'm trying to use V-USB library to emulate a keyboard using Arduino Nano but because of precaution, I don't want to use it's built-in USB port.
My idea is to add another USB port so I cant power my device with the new USB port and use free digital pins as input/output of the new USB serial port in V-USB project.
What circuit I should use and how to add extra USB port to Arduino Nano.

user26638: You can use a USB host mini module for the Arduino NANO and the USB Host Library r2.0

You can implement a keyboard with this...

user20245: USB is deceptively simple.  The underlying protocol is very complex. So a complete answer to your question requires some USB background.
USB is divided into 2 types.  USB Hosts and USB Devices. We are not concerned about USB Hosts here.  USB Devices only need to know about their own functions / features.  If they are a serial port device, like the FTDI chip on the Arduino Nano, they only need to "speak" basic USB protocol and the serial CDC USB protocol.  If the USB Device was a keyboard, it only needs to know the basic USB protocol and the HID USB protocol.
In order to offer up a USB port the Arduino Nano uses an FTDI chip FT232RL.  This chip supports a Serial Port USB Device.  It does not appear to support HID USB Devices.  So your decision to use V-USB is necessary not because of power but because the Arduino Nano's built in USB port supports a protocol other than USB CDC. 

What circuit I should use and how to add extra USB port to Arduino
  Nano.

The circuit suggested is on the V-USB web page appears to contain the necessary information:

Of course you will have to do some research into V-USB software to see which are the appropriate pins to use on your Arduino Nano.

28017: <p>I'm trying to use V-USB library to emulate a keyboard using Arduino Nano but because of precaution, I don't want to use it's built-in USB port.</p>

<p>My idea is to add another USB port so I cant power my device with the new USB port and use free digital pins as input/output of the new USB serial port in V-USB project.</p>

<p>What circuit I should use and how to add extra USB port to Arduino Nano.</p>
 26638: <p>You can use a USB host mini module for the Arduino NANO and the USB Host Library r2.0
<a href="https://i.stack.imgur.com/Gak0B.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Gak0B.jpg" alt="enter image description here"></a></p>

<p>You can implement a keyboard with this...</p>
 20245: <p><a href="https://en.wikipedia.org/wiki/USB" rel="nofollow noreferrer">USB</a> is deceptively simple.  The underlying protocol is very complex. So a complete answer to your question requires some USB background.</p>

<p>USB is divided into 2 types.  USB Hosts and USB Devices. We are not concerned about USB Hosts here.  <a href="https://en.wikipedia.org/wiki/USB#Device_classes" rel="nofollow noreferrer">USB Devices</a> only need to know about their own functions / features.  If they are a serial port device, like the FTDI chip on the Arduino Nano, they only need to "speak" basic USB protocol and the <a href="https://en.wikipedia.org/wiki/USB_communications_device_class" rel="nofollow noreferrer">serial CDC USB protocol</a>.  If the USB Device was a keyboard, it only needs to know the basic USB protocol and the <a href="https://en.wikipedia.org/wiki/USB_human_interface_device_class" rel="nofollow noreferrer">HID USB protocol</a>.</p>

<p>In order to offer up a USB port the Arduino Nano uses an <a href="http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf" rel="nofollow noreferrer">FTDI chip FT232RL</a>.  This chip supports a Serial Port USB Device.  It does not appear to support HID USB Devices.  So your decision to use <a href="https://www.obdev.at/products/vusb/index.html" rel="nofollow noreferrer">V-USB</a> is necessary not because of power but because the Arduino Nano's built in USB port supports a protocol other than USB CDC. </p>

<blockquote>
  <p>What circuit I should use and how to add extra USB port to Arduino
  Nano.</p>
</blockquote>

<p>The circuit suggested is on the <a href="https://www.obdev.at/products/vusb/index.html" rel="nofollow noreferrer">V-USB web page</a> appears to contain the necessary information:</p>

<p><a href="https://i.stack.imgur.com/JXAuR.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JXAuR.gif" alt="enter image description here"></a></p>

<p>Of course you will have to do some research into V-USB software to see which are the appropriate pins to use on your Arduino Nano.</p>
