I'm sure someone will chime in and correct me if I'm wrong, but I don't believe you can change the lens on the Canon S5IS.
Typically, SLR type cameras have interchangeable lenses, but bridge cameras do not.
The S5's lens has a zoom equivalent to 36-432mm. To put this into perspective, if you were to buy an SLR and a telephoto lens that would give you that much reach, you would probably have to pay well into the upper hundreds and possibly thousands for the lens alone.
You will probably not find many cameras with more zoom than the S5, but if you can, you are probably better off just buying a new camera instead of spending your money on converters.