Drafts does not expose native C MultiMarkdown constructs to manipulate the document tree. You can control a number of options with the MultiMarkdown script object, but it does not attempt to wrap the C code in a JS API and probably never will - that level of processing is deeper and would be a lot of code to wrap and maintain (those APIs change over MMD versions, etc.).
There’s no need to copy and paste functions, however. You can use the Include Action step to reuse a script defined in another step, or use require(path) to load it from a file in iCloud Drive to reuse a library of functions.
Not sure what your exact goals are either. You could manipulate the meta data at the top of the content before running it through the Markdown processor pretty easily, relative to try to manipulate those values in the parsed tree like those C function do.