Whole House Humidifier on All-in-One Heat Pump and Freezing Temps
You should be able to install a steam humidifier, and inject steam into the supply duct.  Though to install one, you'll have to have access to the main supply duct inside the home. You'll also as have to have a source of water and power.

Obviously these types of units are available from many manufacturers, and offer a variety of features and pricing. However, in general, this type of unit tends to be more expensive than other types of humidifiers.