[priceless]

Unfortunately in Omnigraffle lines are lines and shapes are shapes. You cannot really convert between them like you are hoping.

That being said, you CAN build custom shapes using the Pen Tool. The Pen tool lets you build custom shapes with fancy bezier curves, but if you don't create the curves, you can easily create shapes just by pointing and clicking in the spots you want to. Try that out.

The other way to make a trapezoid out of a rectangle is to, with the rectangle shape created on your canvas and selected, go to the Style: Lines and Shapes inspector and you'll see a bunch of little thumbnail shapes. You can select a trapezoid there. Not as flexible as building your own, but could be quicker too.

(Also, there are a few "smart shapes" in there like arrows, where you can adjust the shape of the arrow head).

-- Morgan