Use Environment.SetEnvironmentVariable().
string currentPath = Environment.GetEnvironmentVariable("path");
Environment.SetEnvironmentVariable("path",currentPath + ";c:\path_to_libraries");
Keep in mind that this will only be in scope for the current process. If you want to set a persistent environment variable (user or machine scope) use the Environment.SetEnvironmentVariable(string, string, EnvironmentVariableTarget)
overload. See here for that reference.