If you are talking about adding the Vitamin C to a previously mixed and stored, ready-to-go stock solution when preparing to develop film, then yes, as a single, last minute addition with a teaspoon would not be a hassle.

But if you are talking about making a stock solution that uses a teaspoon amount of Vitamin C in the formulation stage, I think I would find it easier to use weight rather than volume since I've set up to weigh all other ingredients.