Any piece of geometry can become a light source in RenderMan for Maya. Select the geometry that should become emissive and click on the Mesh Light button in the RenderMan shelf.
The next time you render, the object will cast light and itself have a constant color which comes from the PxrMeshLight's color and intensity.
When the Mesh Light button is clicked, a new material called PxrBlack is assigned to the selection, and a PxrMeshLight is created and wired into the shading group of the PxrBlack material. PxrBlack acts as a dummy material. It doesn't actually have any parameters, although in the Attribute Editor a link is provided to the associated PxrMeshLight.
For most purposes PxrBlack is an appropriate material, but you can experiment with using others, like PxrVolume. The easiest way to set that up is to begin with the Mesh Light button. Then, select the shading group for the PxrBlack and wire in a different surface or volume material there. This way the area light is already wired in to the shading group.
PxrMeshLights behave much like other lights in Maya. They show up under the lights tab in the Hypershade Window and light linking can be set up in Maya's Relationship Editor.