C can be extended with your own datatypes though. You could create a Roman numeral datatype if you wanted to. You wouldn't have literals, of course, but you could initialize them using string literals. This is how arbitrary-precision numbers work in the C library GMP [1].

So the question is: can you make up new words in Toki Pona? It would seem the answer is no. This would keep the language very simple but also drastically limit your ability to express complex ideas within it.

[1] https://gmplib.org