My girlfriend and I both have iPhones and we both want to use my apple id but have separate numbers. I just bought her an iPad and it only has my phone number under iMessage. Can I add hers to my apple id or the iPad?
Not really except for one special case..
One Apple ID can have two phone numbers, but one phone number cannot be linked with two Apple ID.
If she didn't care to have iMessage work at all, you could sign in to her iPhone with your Apple ID and messages to both numbers would then come to your Apple ID until she decided to start using her number with another Apple ID.