Is it possible to record ADF to FDD on the A600GS?

[F0LLETT]

Or you get one from here, .
Grease Weasle 4.1(with options for case and cables).

Yes you can write stuff back to disk, so real amiga can read it.
However, not sure about Copy protected disks. Doubt something like X-Copy would work for those.
DOS based disks, like workbench, etc etc. will work fine.

[F0LLETT]

that has a Greaseweazle built in and you can connect the floppy drive with a ribbon cable and that´s it 

Indeed, be a nightmare to integrate GW to GS. Then you would have to mess around with placing floppy connector and power. It would just look ugly.