Right way to install custom PHP code in the core vB PHP files

[QSTR]

Hello everyone,

I've made a 'plugin' for my community (couple custom PHP files, some templates and tables)
All is working great BUT.

In order to use this plugin I need to add piece of code to the 'newthread.php' file, in this place (VB 4.1.12 PL3:

271         else
272         {
                      // something to do here
292 
293                 // ### NOT PREVIEW - ACTUAL POST ###
294                 $threadinfo = fetch_threadinfo($newpost['threadid']); // need the forumread variable from this

I've made it via VIM but I think this is not right way to do it (if I use diagnostics, vB says this file is not OK )
So, is there any easy way to install this code exactly in this place via some feature in Admin Panel?

Or if there is no 'easy and nice way' to do this, tell me please if will it be any problem for vB when I leave this code there. (during installation/deinstallation of products? [i know about vB upgrades, when I need to fe. update the newthread.php file])

Thank you.
/sorry for my english/