|Additive manufacturing (AM) has enabled control over heterogeneous materials and structures in ways that were not previously possible, including functionally graded materials (FGM). However, typical computer aided design (CAD) systems - mainly based on boundary representations (B-Rep) - are not built for modeling and representing spatially varying material information, which are the key features for designing FGM parts. While volumetric representations (V-Rep) such as tetrahedral or hexahedral meshes, trivariate splines or subdivision solids can hold the necessary FGM information, they are by default not compatible with existing CAD models. Thus, the capabilities of AM motivate enhancing CAD systems to support volumetric material allocation and distribution. We present an interactive, intuitive and efficient interaction concept based on grading features and transition functions for augmenting CAD models with smooth volumetric material gradients for multi-material fabrication using our system called GraMMaCAD (Graded Multi-Material CAD). This concept has the advantage of being extremely versatile, since the material property functions are left to the arbitrariness of the user, who is free to modify their definition as needed. By providing interactive preview modes, the system allows for efficient exploration of the volumetric gradation of the designed FGM part. We fabricated two functional examples that demonstrate the usability of the system and show the ability to quickly augment models with functionally graded material transitions.