How do I turn a romper into a dress?

For halloween, I am going as a girl from the 1940s. I have had my eye on this red polka dot dress for about 2 months at a vintage store now. Today when I finally tried it on, I found out that it was a romper, so it had shorts at the bottom instead of a skirt. I figured it would be easy to turn into a skirt, and since it was exactly what I wanted, and it only cost $12, I bought it. How can I turn it into a dress without really changing the figure a whole lot?