You might even want to try doing this on your iPhone. What I am suggestions is that you will hopefully be refreshing your account which will let you select the phone number on the iPad, because as Grumpy Gator said, it should appear as a choice once you setup Messages on the phone.
My daughter and I same an Apple ID and her iPhone number is a choice for me to use in the Messages app. The ID is my Apple ID, but the phone is hers.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
Hey guys, I just came across this thread while helping two friends set it up on their laptops, and they ran into the same roadblock, being unable to add their phone number to iMessage on Mountain Lion. After some trial and error, here's what worked for me, from that point on:
1. On your iPhone, go to Settings > Messages
2. Log out of your Apple ID at the top, and then log back in
3. Deselect all of your accounts that are not chat accounts (so if you also use Gchat, you can keep your Google account checked) (I don't know if this step is necessarily required, but it's what worked twice for me; feel free to experiment with this yourself)
4. Give iMessage a minute or two on your laptop, while open, to get the new information. You should get a couple pop-up messages telling you about the new settings. Just click OK for each, and you should be good to go!
Hope this helps, I don't know why it's more difficult for some people, but basically if you just reset the permissions on your iPhone, it should resolve this issue.