You can buy Coaxial SPDIF to Optical converter boxes from Maplin Electronics (
www.maplin.co.uk).
They also sell Optical - Coaxial SPDIF boxes. Both kinds of units are £19.99 each and come with mains PSU.
Alternatively, you could buy a TOSLINK receiver and build a small circuit board.... (see
http://members.ozemail.com.au/~atrac/) for some useful info. Instead of connecting the output of the toslink receiver to a soundcard though, you'd connect it to your decoder or amp.
OR - you could just go the whole hog & do what I've done to my Sony minidisc unit recently - it only had optical out, no coaxial out, so I added one myself. Drilled a hole in the rear of the case, put a phono (RCA cinch) socket in the hole and wired it to the TOSLINK (optical) transmitter module. Works like a dream!
J.