You really need to point your dish at a different satellite if you want to watch French TV. However your Sky Digibox will allow you watch Euronews and Motors TV with a French soundtrack if you choose French as your preferred language in the system set-up.
There are relatively few French channels which are free-to-air on digital satellite, see
www.lyngsat.com for details. You can watch some of these with your Digibox if the symbol rate is 22000 or 27500 symbols/s. However the Digibox isn't capable of driving a motor nor switching between dishes so you would need some other means to do this. In any case it's a bit of a pain to use the "add channels" and "other channels" functions on the Digibox.
A better bet than using the Digibox is a cheap free-to-air digital receiver (about 100 euros when the hypermarkets run a special offer). However it may be more future-proof to buy a digital receiver with a "common interface" (170 euros and upwards) so that you can add a decryption module later.
The French terrestrial channels are all available in analogue on Atlantic Bird 3 at 5°W. You can buy a cheap analogue satellite receiver, dish and LNB for about 60 euros in the DIY sheds like Castorama or Leroy Merlin.