It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.formatting.IFormattedText;

You can always defer an IFormatted Text from a String (the conversion is automatic).

You can add or concatenate two IFormattedText together using the respecting Operators: + ~

Besides that you can use the IFormatter to format an IFormattedText Object.