This is not possible, I'm afraid - sorry
As I'm sure you can understand, the ability to sign packages is restricted to McAfee...
I know this post is old, but Joe is correct, however Anna's implementation is wrong. If you are pushing your own packages to your own networks / systems, you wouldn't sign them as McAfee. You would make your own cert embedded in your updates for the packages you push. Going over that entire scope is way past this thread, but you would sign as YOU, you can't digitally sign as someone else. That's like saying "Hi, I'm Bill Gates," and owning Microsoft.