If you've already got a $ky dish up on the wall, odds are you'll have a quad LNB (the thing on the end of the arm on the satellite dish), or maybe a dual one.
This will depend on the $ky box you have. If you've got $ky+ functionality, you will have two cables coming down from the dish. Just check to see if there are "ports" on the LNB for further cables.
You should be able to connect your Freesat box directly to the LNB on the same dish as $ky, providing that you have free ports available on the LNB, which will be much better than having two mini-dishes stuck on the wall!
If its not a quad LNB, you can pick these up fairly cheaply on e-bay - or you may have one on the new dish that you have acquired, and you could replace the one on the existing dish. A quad LNB basically means that it has connectors for 4 x cables to go into the LNB.
The Freesat website keeps a good track of the equipment that is available:
http://www.freesat.co.uk/index.php?p...dqtrahae1qk8a5
It might be worth having a look on there, and possibly picking one that takes your fancy, and then ask on here what that specific model is like (or search the forums for it).
The Humax range of products does have a lot of fans, and they are reliable, with good picture, but there are cheaper models available, that you might want to consider if you're on a budget.