Well, under a few really critical assumptions, one could indeed multiply probabilities of choice by expected volume by respondent. Some of the assumptions are:
1. The probabilities of first choice from a CBC exercise are proportional to purchase volume across multiple brands (and work for predicting volumes of purchases even though we never asked respondents to do a volume allocation in the choice tasks).
2. The probability of choosing the "None" option indeed reflects the likelihood of picking the "outside good" (either declining to purchase or purchasing from another brand or other substitute category not included in the choice task).
3. The share of volume switches between the None alternative and the other brands in the simulation will adjust properly when adding new products or taking products away from the competitive scenario...even when we potentially did not ask respondents to evaluate choice tasks with the number of concepts (alternatives) matching the number of concepts in the market simulations.
4. Respondents can accurately report their annual purchase volume.
These assumptions are critical to obtaining proper predictions of volume. The scaling of the None% and especially the assumption that one can estimate growth in volume within the category due to adding more brands to the simulation (using choice tasks from the questionnaire that have a constant number of alternatives) are problematic assumptions. For this reason, choice researchers usually prefer to make the assumption that the category purchase volume remains constant.
Respondents are often not very good at reporting volumes or answering choice tasks by stating volumes per alternative. This is another reason that choice researchers usually prefer sticking with the "share of preference" assumption and assuming constant volume (no category growth).