The customization of software process capability/maturity models (SPCMMs) to specific domains/sectors or development methodologies represents one of the most discussed and applied trends in ICT organizations. Nonetheless, little research appears to have been performed on how theoretically sound and widely accepted SPCMMs should be developed to high quality. The aim of this paper is therefore to elicit the state-of-the-art regarding the processes adopted to develop such models and to propose a systematic approach to support the customization of SPCMMs. Such an approach is developed based on ISO/IEEE standard development processes integrating Knowledge Engineering techniques and experiences about how such models are currently developed in practice. Initial feedback from an expert panel indicates the usefulness and adequacy of the proposed method.