That's not how a superseding indictment works! A superseding indictment charges more (or different) crimes than the original indictment. The process for obtaining the indictments in the first place is the same.

That’s kind of the point of the grand jury. It’s an investigative body. Grand juries are typically convened when investigations reach critical mass, and are then used to issue subpoenas for additional information, leading to an indictment, which then leaves to arrest and trial.

Grand juries are often used for a single charge, but can also be used to supervise a lengthy investigation with multiple charges and multiple defendants (like the ones used by Robert Mueller in his investigations of Russian election interference).