A sensor network application requires diverse kernel supports to function properly. With its resource limits the sensor node cannot provide all the functionalities needed by many kinds of applications at the same time. The kernel's functionality therefore requires runtime reconfigurability, which can be achieved via modularizing the kernel. This paper presents a framework that dynamically reconfigures the kernel's functionality according to the needs of the application. In particular, the proposed mechanism handles the address resolution problem of a MMU-less processor. This framework has been implemented on a sensor network operating system, RETOS, which supports multi-threaded programming environments. It efficiently manages the modularized kernel's resources and works in an optimized condition. By providing modularized kernel programming, RETOS optimizes itself with functionalities that various kinds of sensor network applications require.