You can't just add a dummy number, because they are going to mail a PIN to you to verify your address. You have to put the PIN at the AdSense page to complete the process.
Frankly, I don't think you're missing much by not having AdSense. It takes a LOT of hits to make it pay and they are not going to send you any money until you hit the threshold of $100 anyway. There are better ways to bring in ad revenue, IMHO.