The freeware Geochemical Data Toolkit 3.0 (www.gcdkit.org) or, in short, GCDkit, offers a flexible environment for handling, recalculation and plotting of whole-rock geochemical data from igneous and metamorphic rocks. The current contribution demonstrates that the system can be easily expanded by the plugin modules, short yet powerful chunks of user-defined code, which can be easily and freely re-distributed. It describes their internal architecture, as well as the way how they may communicate with the core of the system and can be integrated into its Graphical User Interface (GUI). The plugins in GCDkit provide an appropriate platform for development of modules for numerical modelling of igneous processes. The presented simple plugins for direct and reverse modelling of the major-element mass balance in course of the fractional crystallization provide a sound, and potentially useful, proof of this concept.