Can I combine these two, so that if I'm busy and someone is calling me, they could get a personal message i.e. "I can not take your call right now, you will be connected with one of my colleagues", and then using Call coverage the call is forwarded to another extension ?

It doesn't look like the system has any prompts that play before the call coverage functionality is invoked. You could probably add a customization to handlers but it doesn't look like there is a customization point so if you go down this path you will be modifying system handlers.