[How to] include a code within templates using a plugin or from another template

[Infoman4ever]

Hi,
I know now how to create a plugin and link it to a hook, and how to create a template and put it "manually" on an existing template, but what I don't know yet is how to put a piece of code from a plugin or from a template "created manually" on a specific place within an existing template such as the posbit, posbit_legacy, showthread and so on. I need to do it to create a general product, right?
Well, I tried using hooks, but sometimes they do not help, they do not cover some places within some templates.
Any help will be highly appreciated.