Typically a house with 2 bathrooms will sell for more money than a house with just one. That's the easy part. The hard part is knowing if the cost of adding a bathroom strictly for the purpose of resale will result in a sales price that will not only cover your expenses but result in a higher sales price. That's hard to quantify without seeing the property and your competition on the market. I think it's safe, however, to say that it would certainly add to the marketability of the house.
To get a more quantifiable answer, I would ask a local Realtor that specializes in your area to do a CMA for you looking at homes that have 2 bathrooms vs. just one bathroom.