How do I add wire to a rusted electric box?
I would recommend cleaning the knockouts first. Sand them completely free of any paint or rust. Then scrape clean the circular groove of the knockout on both sides using a sharp pointed tool like an awl.

After doing this, the knockout should come out with a sharp block from a ball peen hammer and screwdriver.

If this does not work, the next step would be to use a tool called a knockout punch set. They can be found in any hardware store and they look like this: