Sort of.
Unless you are replacing sky with freesat, you will need an additional feed from the dish. As you have Sky+, the chances are that you have a quad-LNB (this is the bit that the cables come out of on the dish), with two connections spare.
The freesat box cannot take a satellite feed from the Sky box, so you will need to get an additional cable run from the dish to where the receiver is going to be situated.
You can then plug the freesat box directly into the TV, into either a spare Scart socket, AV socket or HDMI (if you've gone for the HD freesat box).
Keep in mind that nearly all channels that are available through freesat are already on Sky, so if you are keeping the sky box, then the only reason for doing this is if you were buying an HD freesat receiver to get the HD content from BBC and ITV.
Hope this helps...