My next question is, after a defendant is indicted can the prosecutor keep adding to the case just so that they can increase the defendant guidelines? And after the defendant pleas guilty, can they still keep adding to that case, if so is that legal? Because the defendant plead guilty to what's in the case at that they made a plea and not to what was added after the plea.
“After the defendant pleads guilty” - No further charges can be added
Once a verdict is entered (through plea or trial) all matters of criminality around those particular circumstances are closed (barring appeal). In both criminal and civil matters the prosecution or plaintiff respectively is required to bring all matters about a particular event they want dealt with to trial at the same time. The legal principal is called res judicata or issue estoppel.
As per this question - additional evidence can be introduced for sentencing purposes
So, the prosecutor is perfectly capable of adding charges against the Defendant. Consider the hypothetical example where a previously convicted felon, Charlie, pulls out a gun and starts shooting, killing Bob and Wounding Alice. The prosecutor would charge Charlie with 1 Count of First Degree Murder (Bob's murder), 1 Count of Attempted First Degree Murder (Alice's Wounding), and 2 counts of Unlawful Possession of a Fire Arm in the commission of a violent crime (As he was previously convicted of a Felony, Charlie cannot own any gun legally). While the case is going through pre-trial motions, Alice dies due to the gunshot wound. The prosecutor can change the charges to 2 counts of First Degree murder, and drop the one count of attempted... since this is now a murder victim*.
Prosecutors will also get them on any charges they can manage, so long as they are not "Lesser Included Charges" (i.e. The Prosecuter cannot charge Charlie for Alices Murder and the Attempted Murder of Alice, because in order to Murder Alice, Charlie would have to attempt to murder her. The prosecutor could still charge him from illegally possessing a fire arm because that's unrelated to him killing someone), as the Jury can convict on any one of those charges too. The more there are, the more likely Charlie is getting put away for some crime.
For your second half of the question, suppose David is caught in a sting selling illicit drugs to Emily. He has a record of convictions for this and is also found with a fire arm (as a felon, he can't own a gun. Period). David is initially charged with 1 Count of Possession with intent to sell and One count of Unlawful Possession of an Illegal Fire Arm (he did not pull it on Emily, he just had it on his person when arrested).
In this situation, David is a low level pusher and gets the drugs from someone else to sell on the street (lets call him, Mr. Kingpin for now). Taking David off the street won't do much as Mr. Kingpin will find another person who wants to make Bank on David's territory and supply that guy... But if they can arrest Mr. Kingpin, they can put all of his pushers out of the picture. The prosecutor offers David a deal: If he turns states' evidence against Mr. Kingpin, the Prosecutor will drop the charges to Possession of illicit Narcotics (without intention to sell) and drop the gun charge completely. This is enticing because the Prosecution has David dead to rights on the charges he is facing, both of which carry a heavy jail sentance... the prosecutor would better spend tax payer resources taking out a bigger chunk of the cartel than David and will need David as a witness. David pleas guilty to a lesser sentence and still goes to jail, but for a reduced time, and the Prosecutor shuts down more of the cartel than a pusher on the street AND Saves money on prosecuting David. Win Win.
If David takes the plea and pleads guilty in court, the trial is effectively over and the Prosecutor cannot charge related to the specific event of the sale of Narcotics to Emily (if he sold to her last week, it's still game... but the night he was arrested cannot be charged for any newly revealed evidence pointing to a new crime related to the sales of drugs.
Edit: In cases where the indicted person was extradited from another nation to face charges, the defendant can only be charged with crimes that he was extradited to face. If a new charge is to be brought, it must be re litigated with the country that extradited the accused in the first place.