O.k. so I downloaded the lua-archive from morphos-files to get the needed includes (or are there newer ones to be found elswhere?)

Newer ones are in the current public MorphOS SDK...

Now, what I would like to do is to call that function from elsewhere. So another program starting that script (like LuaX) or even another program calling it directly with lua.library.

I don't really get the problem. If you have a Lua script in a file, any number of Lua interpreter instances can load and execute this file.