The C70 convertable comes with a factory installed subwoofer to try help overcome all the intense road noise (which never ceases to amaze me whenever I have to test drive one) BUT sure, you can probably order one from a dealers parts department (never been asked to install one yet) and I'm sure it won't come cheap.
If you want a really good subwoofer, I'd advise buying one (with a good sub amp) from either Bestbuy, Circuit city, Crutchfield or Ebay (once you know which one you want). It'll be cheaper and sound better too.

As for the reversing alarm, although many newer cars have them I'd never pay extra for such a device as I doubt that it'll be all that usefull. It's a device made for people who can't judge the rear bumper distance to another car IMO.