Hello! In order to mod CS the way you're thinking, you need to have its whole source code. All of the weapons, gamemode rules etc. Unfortunately, there's only an official SDK for Half-Life, and an unofficial OpFor SDK, nothing for CS.
So, the best thing you can do is to replicate CS 1.6 in a HL mod, which is a respectable amount of work, annnd yeah, stuff like adding new weapons requires a good bit of C++ and HL SDK experience.