Not correct, sorry. CBC utilities are scaled such that their average is zero. So, adding a new level with a utility value of zero is to say that this new level has the average preference of the other levels. If you didn't include a "not offered" level in the study to begin with, it's hard to know how to figure its utility.
I once was asked by a client to do this, many years ago, when we received a project that had forgotten to include the null (assumed worst utility) level. We decided to impute a value per each respondent's HB score equal to the worst utility level for that attribute for that respondent, minus some small utility amount such as 0.1 or 0.5. We warned the client of the assumptions we were making in that imputation.