I think the best way to add them is just to create a shading object. you can create just a shading surface that will influence your result. Have a look at This Video. This will show how you can model surrounding buildings etc.

Hope this will help.

Regards Waseem

Just as an aside, it is absolutely crucial to include local ground geometry when doing daylighting calculations, within reason. Your building of interest will shade the ground adjacent to your windows, and this shading affects the daylight availability (and does so in a variable manner throughout the year). When I say "within reason", a 1:5 ratio of window head height to edge of local ground plane is a good rule of thumb. Beyond that distance, the ground reflections are not influencing the daylight through that window, and ground geometry past that boundary will only serve to decrease the resolution of the ambient calculation in Radiance. Similarly, vertical geometry that can block direct sun and skylight from a window's view, while important, should be reigned in closer to the building model perimeter by way of using scaled "impostor" geometry if possible. This tactic can also serve to simplify the exterior geometry in terms of polygon count, which EnergyPlus would be happy about. =)