Can I plug a six-prong outlet extension into my generator so I can run a few 110-120 small appliances?

As long as you do not exceed the power rating of the generator or its specific outlet, expanding the number of devices this way is fine. Keep in mind that increasing the number of devices also increases the chance that an electrical fault could occur, necessitating a protection device of some kind. Many generators include protection, but a power strip with additional protection may be wise. You also need to be sure that the cabling and connectors used are designed to handle the amount of current that would flow when all of the devices are on.