Dirt mapping in IRender nXt - how?

Hi

One of the best ways to achieve photorealism is to have dirt mapped materials so that the diffuse channel of materials is not a flat uniform colour. For example a white walled building will have stains from rainwater near the top edges. In v-ray and other 3d programs like 3dsmax dirt mapping can be done either natively or through plugins like 'rich dirt'.

Are there any users out there who can describe how similar effects can be achieved in IRender nXt?

Thanks in advance.