It is possible with a fixed mesh or fixed/motorized solid prime focus dish to receive both C band and KU band.  You'll need one of these to do the job right.  The MTI Turbo 4200 LNB is a dual band LNB capable of receiving signals on C band between 3.4 and 4.2GHz as well as KU band between 12.250 and 12.750GHz.  This dual band LNB has 2 output ports (one for each band) and these ports can be treated like 2 seperate LNB's so you can combine the signals together using a DiSEqC switch.  Then a your existing coaxial cable can be connected to the switch.  These LNBs have been sold by us for over 5 years and have proven themselves to be reliable.  Beware of imitations.