Building your own twin sticks from hacked DC pads was discussed at length on this msg board a long time ago (I can't remember if it was last summer, or the summer before that). But I do remember it was before Planetjay Upgraded us to this new php/mysql driven msg board.
The outcome of the discussion was that you needed to create a custom circuit to allow the dreamcast pad to mimick all the directional combinations of the real twinsticks. Somone was going to make it, but we never herd back (he probably went insane

). I also believe it wasn't going to be 100% accurate (I think there was some movements that couldn't be duplicated by the dc buttons).