I think you can go the other way (PSX->DC) with a few commercial products, but not the way you want to go. Among other things you would run into problems with the number of buttons - the DC pad has too few.
Just having the connectors won't help you much, the controllers will still speak with the wrong protocol. The commercial converters contain a PIC or other chip to translate the bus protocols into a format the console understands.
you cannot use guncon on dc, light guns wont work.. depending on the adapter you may be able to use steering wheels, and arcade sticks.. dance mats and saturn VO twin sticks too.. i have been wanting to find a xbox or dc to playstation adapter, i hate sonys crap