They can.Well... I'll do some reading, but this used to be true. Though, admittedly, it may have changed.
"One way an NFL team can free up salary cap space without releasing players is to do a contract restructure. This only works if the player has more than one season left on their contract unless the team is willing to add void years to the end. If the team adds any more seasons that include a base salary, it would be a contract extension, not a restructure."
Cannot add years but IN MOST CASES, you can convert signing bonuses without player consent.