Yes, it is, but the main reason they're called "internal" is that handling of those internal commands is literally part of totalcmd.exe itself. So in order to change their behavior you would need to obtain Total Commander's source code, edit and recompile it.
Unfortunately, Total Commander is not open-source, so your only options are:
- see if its plugin system allows adding custom commands, and if so write a plugin;
- see if the same task can be done by running an external tool;
- contact the author with a feature request (or bug report);
- use reverse-engineering tools to patch the compiled .exe directly (a bit difficult and probably not very legal, considering WCMD's shareware status).