Do you mean that you want to get a Sky multiroom subscription and then connect that second box directly to your Mac?
If so, be aware that the second box won't be able to access any of the programmes which are recorded to your Sky+ box. It will need an additional wire from your satellite dish to your bedroom and the second box will also need to be permanently connected to a telephone line.
In terms of connecting the box to the Mac, you'll need something like the
Elgato EyeTV 250 Plus which has a composite video input. Again, this won't provide perfect picture quality (the best connection for a standard definition Sky box is RGB SCART, the big thick black cable and as far as I know there is no way whatsoever to connect this to your Mac) but it will be good enough to watch. The Sky box will not include a cable suitable for connection to the EyeTV, so you'll need to buy one of those as well.
Personally, I don't think the expense is worth it. If you only want to watch BBC One, ITV, More4, Film4 etc then you would be better off with a something like the
eyeTV Hybrid which you connect to your rooftop aerial and which will get all the Freeview channels.