Create a macro that appends some code an existing command, with arguments

As I am working on improving my answer to Different approach to literate programming for LaTeX, I have created a macro that automatically processes two arguments and formats indexes accordingly. The arguments are strings (not control sequences), with #1 being the type of macro and #2 being the name of the macro.

Here is a sample of the macro I am using. It relies on etoolbox and xstring.

\newcommand{\set@macro@type}[2]{    \StrSubstitute{#2}{@}{"@}[\entryname]    % Default definitions    \def\macro@type{#1}    \def\macro@format{#2}    \def\macro@index{#2@\string\texttt{#2}}    % Specific type definitions    \ifstrequal{#1}{m}        {\def\macro@type{}         \def\macro@format{\textbackslash#2}         \def\macro@index{%            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}    \ifstrequal{#1}{l}        {\def\macro@type{length}         \def\macro@format{%            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}    \ifstrequal{#1}{e}        {\def\macro@type{environment}         \def\macro@format{\textbackslash#2\par\textbackslash end#2}         \def\macro@index{%            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}}

However, I would like to make it so that the user can add new types of macros with an interface such as:

\addmacro{<name>}{<type>}{<format>}{<index>}

which should append the following code to my existing macro:

\ifstrequal{#1}{<name>}    {\def\macro@type{<type>}     \def\macro@format{<format>}     \def\macro@index{<index>}}

Ideally, the \addmacro command could be starred, too, as I have an additional switch to toggle depending on the user's wishes.

So, my question is: how can I implement a macro that appends some code at the end of the main macro, and that can be used repeatedly. I would prefer to keep using etoolbox if possible (I have seen \apptocmd in the documentation but can't get it to work). The main difficulty which I am encountering is with the arguments #1 and #2 of \set@macro@style, which the user must be able refer to in some way. How could this be implemented? An explanation would be very welcome too.