I asked Russ Graf and this is his reply:
It is true that SIA does not offer Customer-level forecasting as a standard feature. However, there are couple of techniques that can be deployed by us to often solve the problem in specific situations. For example, in many cases the user may only need to accommodate the forecasts of a few large customers, like a Walmart. In those cases we can often define a logical warehouse to align those forecasts to and get the desired result with little inconvenience.
In other cases, the users wishes to roll up many individual customer forecasts and include those in the SIA overall forecast. In that case, we can devise a methodology to import those forecasts from Excel and again achieve the desired result.
Having said that, it has been the experience of our world-class replenishment consultants that the ""need"" for customer level forecasts is often overstated, and our first questions on the topic usually start with Why? and Do you really? So it is best for us to have a conversation about the specific needs of any user rather than assume we cannot meet a requirement.