Yes, instead of extending the alphabet we can extend the string length boundary. We also can keep alphabet and string length boundary constant and extend the representation time boundary. All of these allows to extend the largest representable number. However, if all three are fixed, I think a largest representable number exists.
If we allow for logical paradoxes to be representations, then there is a simpler way to represent any number, e.g., "The number twice as big as the largest number we thought was possible."