Is it possible externally? Yes.
Is it good externally? No.
Inb4 "Why"?: To achieve the so called "anti aim" you have to write your weird angles to the angle buffer that the server and every other player receives while your viewangles that you see stay the same. This is done by writing to the usercmd structure ( a structure which is passed to CreateMove and later on sent to the server ). Internally you can simply hook CreateMove and have access to this structure, externally you have to find the location of where these buffers ( multiple of them ) are stored. The problem is that you have to hit the right buffer ( the one that gets sent next ) + the right time, or else the game could be overwriting them after you've written to them. This won't always work.