Haolong

Hey guys, For a few weeks i downloaded the Modpack Hexxit (MC version 1.5.2) through the Technic Launcher and played it for a while. Even if it is a lot more interessting than Vanilla Minecraft, there are still a bunch of things i would like to change. So i watched a lot of tutorials about how to write and integrate mods into vanilla minecraft with MCP and Eclipse. So far so good, this isnt really a thing anymore. The problem now i am facing is, that i have no clue how to change existing mods. Is this also possible with the MCP and Eclipse ? Is it even possible ? And is it even allowed? I would appreciate any help or links to turotials Tizian