I'm afraid to say that this idea is hardly original. This has been discussed before and there are several threads on ZIF connectors, covering the various problems encountered with attempting to fit them into iRivers (both H1xx and H3xx series). I doubt that a ZIF drive and ZIF->IDE adapter would physically fit in the space of a 1.8" IDE drive, hence stolltech's decision to hardwire the connector to the mainboard.
You shouldn't need to speak German to understand that pdf. The schematics showing the pin locations are in English and the pinouts of the two interfaces show what number pins should be connected together. That should provide enough information to build an adapter. Then again, there's always
Babelfish if you really want to know what the rest means.
That Japanese site is only for the connectors to build the adapter, as described in the other thread on this topic that you posted in.