I want to buy a MacBook but I want to know if I can add WiFi connection in it ?

Adding Internet service is a separate process regardless of which Apple notebook you decide to buy. Regardless, all Apple notebooks come with Wi-Fi wireless capability to connect to a wireless network. Whether or not you will get Internet access will depend if that network has been provisioned with an Internet service.

Note: The MacBook was discontinued last year.

Internet service via WiFi is standard on all Macs.