I think that you are looking for the drape command. Chose a view from top, and draw a rectangle on top of the polysurface, drape creates a single surface using the information of what is it under the rectangle you draw.
I hope this helps.
seem to be can not do. actually we can use merge all face command. but as I saw. I can not apply this command for bending surface.
did you get the solution? I also same your topic. if you had solution. can you share with me?