CLISP can be easily extended the same way any other Common Lisp implementation can: create a lisp file with your variables, functions, macros, etc.; (optionally) compile it with COMPILE-FILE; LOAD it into a running CLISP, and save the memory image.

This method does not work when you need to use some functionality not available in CLISP, e.g., you want to call a C function. You are urged to use External Modules instead of adding built-in functions.