You cannot flatten surfaces with double curvature in Solidworks. SW can only flatten single curvature if you do it using sheet metal functionality.
I have done FEA for an approximation. Set up contact with a rigid plate and increase gravity or a pressure load until it is flat enough. This is sensitive to Poisson's ratio though so vary that to make sure your results are accurate enough for your application. I hope this helps.
We have a number of Add-ins that SW has partnered with to provide flattening - BlankWorks, LogoPress and 3D quickpress. You can check them out on our Partner page at the SW website.