I'm curious about this as well. I've certainly googled and found cases where additional charges were later added, but I'd love to read a discussion the legality

I'm curious about this as well. I've certainly googled and found cases where additional charges were later added, but I'd love to read a discussion the legality of using the grand jury process to discover evidence for charging people not named in the first indictment. (https://www.justice.gov/opa/pr/superseding-indictment-boyd-matter-charges-defendants-conspiring-murder-us-military-personnel https://www.justice.gov/opa/pr/superseding-indictment-boyd-m...)