Unfortunately, that functionality is limited to XNA resources, so won't work in a XAML app.
Using physicshelperxaml, you could create a Path element that outlines the bitmap (you would need to draw this out manually). First put the image inside a Canvas element. Then draw a Path inside the same canvas and make sure the Path is the
child in the Canvas. Set the Canvas Visibility to Collapsed so it isn't seen by the user.
The older "Physics Helper" library had code that was similar as the texture to polygon you're mentioning, but it was a bit slow and I never ported it to the newer Physics Helper XAML library. But if you're interested in dissecting that code, it is