There's no direct way of loading script contents into a global table.
You have two options, one of which is easy, and the other requires too much knowledge about the script on the C/C++ side.

One option is to change your script so that the function and all other properties you want are already defined as being in the table, like so:

loadedScript = {}

loadedScript.helloString = "Hello World"

function loadedScript.fwrite(fmt, ...)
    return io.write(string.format(fmt, unpack(arg)))
end

The other way requires that on the C/C++ side you know the contents of the script, and is a bit longer. You manually create a table, and assign each of the values in the script to be a part of that table, something like this:

//functions used:
//http://pgl.yoyo.org/luai/i/luaL_dofile
//http://pgl.yoyo.org/luai/i/lua_createtable
//http://pgl.yoyo.org/luai/i/lua_pushstring
//http://pgl.yoyo.org/luai/i/lua_gettable
//http://pgl.yoyo.org/luai/i/lua_settable
//http://pgl.yoyo.org/luai/i/lua_pushnil

//first run the script file so you get the values inside into the global table
luaL_dofile(L, "your_script_file.lua");

//now that everything in the script is in the global table, create your loadedScript table
lua_createtable(L, 20, 20);

//then get the existing fields from the global table and push them into the newly created one
//first, push the name of the field twice (we need it for later)
lua_pushstring(L, "helloString");
lua_pushstring(L, "helloString");
//then get the value
lua_gettable(L, LUA_GLOBALSINDEX);
//then, set the value to be part of the new table
lua_settable(L, -3);

//now repeat for the function
lua_pushstring(L, "fwrite");
lua_pushstring(L, "fwrite");
lua_gettable(L, LUA_GLOBALSINDEX);
lua_settable(L, -3);

//optionally, you can remove the global versions by setting their value to nil
lua_pushstring(L, "helloString");
lua_pushnil(L);
lua_settable(L, LUA_GLOBALSINDEX);
lua_pushstring(L, "fwrite");
lua_pushnil(L);
lua_settable(L, LUA_GLOBALSINDEX);

As you can see, this way you require complete knowledge of the script contents in the C++ world, since there is no way to know what variables the script contains (at least not a practical way which doesn't involve writing your own parser which generates an intermediate script file that looks like the first option i gave).