Aspect orientation is important not only at the programming-level but also at the modeling-level. We previously proposed an aspect-oriented modeling language called AspectM for managing modeling-level aspects. Although AspectM provides basic modeling facilities for a modeler, the language constructs cannot be extended. In this paper, we propose a mechanism called metamodel access protocol (MMAP) that allows an application modeler to access and modify the AspectM metamodel. MMAP consists of metamodel extension points, extension operations, and primitive predicates for defining pointcut designators. MMAP enables a modeler to represent application-specific crosscutting concerns.