You can do that by stretching the paper first. Saw a technique on TV that works well.
1. Place watercolor sheet on gatorboard.
2. Spread water all over sheet with wide brush.
3. Turn it over and do the same.
4. Turn it over again, and spread more water.
5. Wait about 15 minutes, then place single sheets of paper towels over it.
6. Use another set of paper towels placed on top of 1st set of paper towels, push down with your hands from middle out to push any remaining water out to sides. Start at top and work down. Remove towels and let sheet dry.
7. If when dry it still curls a bit, repeat.