Thanks to a suggestion of Andrew Sterian, ePiX is extensible. The preferred method is to create external modules for use at run time, analogous to a LATEX macro/style file. It is also possible to modify the ePiX source code itself before compiling; this is inflexible (and requires administrator privileges), but can be useful for changing system-wide defaults. User extensions span a spectrum, from header files that require only basic knowledge of C++ to separately compiled libraries that substantially extend the capabilities of ePiX.