What you're thinking of is "depth of field" commonly referred to as "Dof" and not motion blur. And looking at the particles, it seems like they're motion blurred already.
Dof can be simulated a few different ways. For the first two you will need to whip out Maya's distance measurement tool and get a reading from your camera to your subject(Create>Measure Tool >Distance Tool). Lets call that number X distance.
1. The Depth of field attribute on Maya cameras which is the most obvious way, but it's very dependent on your AA sampling in the render settings to eliminate any grain.
2. If you're using Mental Ray, you can plug either one of MR's dof shaders into one of the camera's lens shader slots - "physical_lens_dof" or "mia_lens_bokeh" ( I prefer this one).
The camera will need the X distance value so it knows where to focus, if you're using method 1. its the "focus distance" and for method 2, it would be the "Plane" attribute.
3. Render out z-depth pass or passes and use that to control a blurring filter in a compositing program.