Hello dadreflog,
Thanks for asking about adding a number in iMessage on your iPad here in the Apple Support Communities. I understand you are wanting to sign into the iPad with another or second phone number. Let me offer some guidance to help you get this done.
The first thing we want to ensure is that all your data is safe. You can make a backup in iCloud or iTunes. Use this article for help with that: How to back up your iPhone, iPad, and iPod touch.
Then, to get multiple numbers to sign into your iPad under iMessage, the first thing that would need to happen is that you would have to be signed into two separate iPhones with the same Apple ID. This would attach your Apple ID, to both phone numbers. This is described in the help article here: Add or remove your phone number in iMessage or FaceTime on your iPad, iPod touch, or Mac in the first section titled "Set up or remove a phone number".
Once that is done, please scroll down to the section titled "Turn your phone number on or off for your iPad or iPod touch". If you have gone through that section and are still experiencing issues, please ensure the settings are correct on both the iPhone, and the iPad. What you need to check is described also in the same article, under the section titled "If you don't see your number in Messages or FaceTime on your iPad, iPod touch, or Mac".
Thank you again, and have a great rest of your day.