Though not recommended as it involves jailbreaking, you might try these steps:
Step 1: Download and install the iPhone Configuration Utility from the links given below.
iPhone Configuration Utility for Windows http://support.apple.com/kb/DL926
iPhone Configuration Utility for Mac http://support.apple.com/kb/DL851
Step 2: Connect your iPad with your Mac or PC.
Step 3: Download WhatsApp for iPhone on your PC or Mac using iTunes. http://itunes.apple.com/us/app/whatsapp-messenger/id310633997?mt=8
Step 4: Now launch the iPhone Configuration Utility. On the left hand side, you will see your connected iPad; click on it.
Step 5: Click on Applications from the tabs you see at the top of your screen.
Step 6: Click on Add at the top left and browse for the WhatsApp IPA file you download in Step 3. It can be found under the iTunes Media > Mobile Applications folder.
Step 7: Scroll down and click on Install. WhatsApp will appear on your iPad’s Springboard.
Step 8: On your iPad, open up Cydia and install WhatsPad tweak from the BigBoss repository.
Once the WhatsPad tweak has been installed, you may launch the WhatsApp app from your homescreen and it will work the way it works on your iPhone. Also keep in mind that WhatsApp will ask you to enter a 3-digit PIN for activation which you’ll receive in a text message, therefore be sure to activate WhatsApp using your active phone number on which you can receive text messages in order to get the PIN.
If you want to scale it up and take the advantage of every pixel of the iPad’s screen real estate, you can do that as well by downloading one of the many well-known tweaks from Cydia, such as RetinaPad or FullForce.