Ice maker leaks can lead to huge damage repairs. Some replacement tubes come with a saddle valve that looks like the one below:
Designed to work with a copper pipe, the valve will pierce the tubing. But this product is designed to fail. This simple white metal will rust causing a leaks and in many states, these are illegal. Home owners can purchase them, but license plumbers will not use them.
To go from a saddle to a tee, cut out the section where the existing saddle valve is, this removes the hole, and install the copper tee. You will then stub out from the tee so that you can attach a 1/4 turn ball valve or angle stop. From there, you plumb to the humidifier. Make sure when you are buying all these parts that you have the correct threads from the valve to the humidifier line.