Nexus 7 doesn’t have GSM facility. So that means installing Whatsapp on N7 shouldn’t be possible. But in my experience despite of it giving warning it installs straightforwardly. Here’s my exprience on how I migrated WhatsApp from my phone to Nexus 7.
1. Install Whatsapp on N7 from market or through APK file. But don’t open the app yet.
2. Create backup of existing Whatsapp messages:
If you don’t want to migrate existing messages then you can skip this step. Of course, there is no reason why you should skip, since its pretty easy. Just transfer the file: “msgstore.db.crypt” from phone to tablet, which resides at “/sdcard/WhatsApp/Databases/”. And keep the file in the same folder in N7. Create the folder database if it doesn’t exist.



3. Now open WhatsApp on N7. Following screen should popup:

4. Enter the phone number of you phone with which WhatsApp was configured previously.
5. Since tablet don’t have messaging facility, wait for following screen to over. After which you should receive a WhatsApp numerical code on your phone. Should be 6 digit.

6. Enter the 6 digit code which you receive on your phone to following screen:

7. As soon as you enter correct code, activation should began and should be activated within seconds. After which WhatsApp will pickup the database file which you kept at Databases location.

8. Click Restore. And all the messages from backup will be restored and you will get a confirmation message:

9. Tap continue, and voila, you just finished installing and activating WhatsApp on your N7:

10. Enjoy.

And oh btw, Whatsapp activated with same number can only work on one device at one time. So you will have to re-verify if you want to use it again from phone:

 
Bonus: You can even install Windows XP on N7.
But… its so slow that its completely useless. Slowness is not due to the device but the limitation of the virtual machine on which it will run. But if you really want to make iOS users jelly, you can head over to this thread to do that. Its pretty much simple, but requires you to have a WinXP ISO image, preferably made light by nLite.