Adding new results during review process
Yes, yes it is. Just make it clear when you do so the reviewers can review the new additions. 

That said, if you think the result merit a new paper, make a new paper, and reference the one under review.

However, while I am an early stage researcher my suggestion is: have a high ethos when publishing. It is way better to have 1 very good paper than 3 weak-related ones. Your objective is to show the world new research, do not fall in the paper-mill trap. Your worth is in the quality, not in the quantity.