I have porch steps where the bottom step has a 9-1/2" rise while the rest have a 7-1/2 " rise. The run is 11". Can I add a new bottom step or platform with just a 2" rise so that the next step will now have the normal 7-1/2" rise?
If I understand the question correctly, then steps should always be a uniform height. The 9 1/2" rise at the bottom of a series of 7 1/2" rises is definitely bad.
Adding a 2" platform on the ground below the step to even out the overall step size is a great idea. I'd suggest you make the platform be large - something like 2 feet deep and 6" wider on each side than the step itself. That will give it a clear, distinct landing feel and be much safer than having that 9 1/2" drop at the end.
If you can't do that, however, then making the new bottom step be only 2 1/2" above ground is better than the 9 1/2" you have now.