Disclaimer : This question pertains to realizing a non-mechanical, very specific thing (within the scope of Adventurers League) for goofy purposes. You may take it with a grain of salt.

A Tundra Storm Herald Barbarian (subclass from Xanathar's Guide to Everything) gets the following Storm Soul secondary feature at 6th level :

Moreover, as an action, you can touch water and turn a 5-foot cube of it into ice, which melts after 1 minute. This action fails if a creature is in the cube.

Let's assume that the secondary feature lets you make a smaller cube than 5 foot (if you have access to less water than that).

A Decanter of Endless Water is an uncommon Wondrous item that lets you shoot water from it :

This stoppered flask sloshes when shaken, as if it contains water. The decanter weighs 2 pounds. You can use an action to remove the stopper and speak one of three command words, whereupon an amount of fresh water or salt water (your choice) pours out of the flask. The water stops pouring out at the start of your next turn. (goes on to list the three options).

The goal here is for the Tundra Barbarian to "shoot ice" instead of water (no added mechanical effect), without external help (someone else to activate the Decanter for them). This is not so easy since the Decanter's water finishes pouring at the start of their next turn, right before they can freeze it "mid-air" with their Tundra feature. Action Surge would possibly enable it to work, but I don't have that as a pure Level 20 Tundra Barbarian.

To that effect, can a Tundra Barbarian pre-emptively freeze the water contained inside a Decanter of Endless Water (in order to shoot ice upon activating the Decanter) ? (for example, by removing the stopper without saying a command word, and reaching his hand inside the device to freeze water, which may or may not be possible)

No, there is no water in the flask

The Decanter (DMG, 161) description heavily suggests that there isn't any actual water in it when not in use(emphasis mine):

This stoppered flask sloshes when shaken, as if it contains water.

The use of "as if it" suggests that the item itself is just a decanter. There is no water in it when not in actual use, and the water it decants is magically created when you:

...use an action to remove the stopper and speak one of three command words, whereupon an amount of fresh water or salt water (your choice) pours out of the flask.

The fact that it also may be fresh or salt water also supports the idea that there is no water prior to the command word, and the command words create the water as it comes out, but nothing is actually inside it.