Can I switch from Arduino IDE to Atmel studio or Visual Micro but still use Arduino library? If switch from Arduino IDE to Atmel studio or Visual Micro, how to use existing Arduino library? 

Library serves important function of 'packaging' real and complex hardware (like NRF24L01 radio board) into functions of 'relatively standard format', like radio.init, radio.write, radio.read, radio.on, etc.  

This allows user to easily use the hardware without reading the data sheet. 

How can the same be achieved under Atmel studio?
At the end of the day, an Arduino library and the Arduino core are just files full of code.

There is nothing to stop you creating a project in Atmel studio (or wherever) and importing all those files into your project.

You will need all the core files (hardware/arduino/core/arduino/*) plus all the files for the libraries you want to use.