Is it possible to do a Master's in Mathematics after a Bachelor's in Economics?
If I were in your position, I would look at the requirements for admission to Master's programs at universities you're interested in and see if you can find a way to fit their requirements into your required curriculum. 

For instance, here are the admission requirements at my university. I'm not sure if it's top-10, but definitely top-20. At this school a degree in math is not required for admission to grad school. It looks like they require up to an analysis course and some abstract algebra. This is likely only a few classes beyond your required calc+linear algebra classes for your economics degree.