Unfortunately it is not available to iPad users though iPhone users can use it. This is because WhatsApp requires a phone number. In the following method you can install WhatsApp on iPad without iPhone or jailbreaking. You will need a Mac or PC though to follow this method.
Furthermore, can you add someone on WhatsApp without their phone number?
Yes. Just open the WhatsApp application on your phone and tap “Contacts” then tap the “+” sign. Bear in mind that WhatsApp uses exclusively mobile numbers in order to find users. This means that if you add a contact to WhatsApp without a mobile number, you will not be able to contact this person via WhatsApp.