Almost all thermostats have a FAN switch, that can be set to ON or AUTO. In the AUTO position, the blower comes on when the furnace needs it (during heat/cool calls). In the ON position, the blower will always be on. This is usually controlled by the G terminal on the furnace and thermostat.
I'm not sure exactly what you're trying to do, so I'll cover a couple different setups
Blower on when you want
For this situation, simply connect the G terminal on the thermostat to the G terminal in the furnace. When you want the blower on, set the thermostats FAN setting to ON.
Blower on when house is too warm
If you want the blower to come on when the temperature in the house raises above a set temperature, you'll simply connect the Y terminal of the thermostat to the G terminal in the furnace. This way whenever the thermostat calls for cool, the blower fan will come on.
Depending on the furnace, you may be able to connect the Y terminal on the thermostat to the Y terminal in the furnace, and simply not connect any control wires from the furnace to the A/C unit. Most furnaces activate the blower after a predetermined (possibly programmable) delay, when the thermostat calls for cool.
Blower speed (Multi-speed blower)
Blower speed may vary based on the different functions. For example, heat might be medium low, fan might be low, while cool may be high speed. So you'll want to take that into consideration as well.
Without more information about your specific equipment, and what exactly you're trying to accomplish. It's difficult to provide more accurate, specific information.
The switch is not required for proper operation. I believe it's intended as a fail safe in the event the humidifier gets turned on without the HVAC fan enabled. In this scenario, the humidifier could theoretically flood the duct. If wired properly, I can't think how this scenario would occur, but I suppose a specific type of system failure could.
Best Answer
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.