yeah if you don't want to model everything in 3D then your best bet in photoshop is to draw them in by hand.

When I want a 3D object with a shadow and I don't want to model everything just to have the shadow, as the case of these buildings, I will model the hidden structure with just simple blocks and cylinders. At these kind of scales it creates enough of a realistic shadow that it works for me.
One other thing I do since I don't want to render in the ground for the shadow to appear on because I want to be able to put the item on any kind of ground - In 3DS Max there is a matte/shadow material which I can put on a ground plain. If I render to a file type with alpha channel that will put my object on transparency with a semi transparent shadow. Its the cats meow for work like this.
I have no idea if sketchup has anything similar, but its worth looking into.