How do I increase the capacity of a small humidifier?
If I understand your question and ascii art, what you're looking for is called a bulkhead fitting.



There are a ton of different types: made from different materials, inside thread, outside thread, barbed, one-sided, two-sided, and all different sizes. I'm sure you can find one that is suitable for your application. The big box stores should at least have a few to select from, your local plumbing supplier will have more, and of course you can find anything online.