Hello @danyloh!
Unfortunately I believe that the only way to currently expand the tree is with values that are pre-defined in the array which you give the component. But these values can be dynamically set with strings from other components!
I was just testing things out and you can definitely pass in the value of a string typed into a text input inside the expand array and it will dynamically expand to that checkbox.
Let me know if that works for you!
If you want to expand every single option:
You could try running a JS Query block to 'flatten' out the object which is the source of the checkbox tree to create an array of all the keys. This final array of keys could be saved in a variable and you can set the expand array value in the tree component to this variable.