Can I use a humidifier to improve my symptoms of a lung infection?

Not beneficial. Humidifier is basically to provide extra moisture to the upper airway, i.e., nose and throat.
Humidifier for Lungs. Your airways love warm, humidified air, coupled with frequent forced coughing to expel mucus. A mist humidifier or long, hot shower works well. If you can't expel the mucus, forced deep breathing, coupled with chest percussion may bring up the mucus.