Is it Poser or ZBrush that is decimating the OBJ?

I would suspect that a raw scan would be way too many polygons for Poser to handle (they are usually in the millions). And typically when you reduce the polygon count in ZBrush (either by manually or automatically retopologizing the high res mesh), normal maps are created by projecting the high resolution details on the low resolution mesh.

It might help if we know more details about the scanned object (how many polygons, etc) and which app is doing the decimation

This typically won't be something that would be handled with GoZ, because that is usually a round-trip function (Go Z out from Poser to ZBrush, and then GoZ back from ZBrush to Poser). The most likely approach would be to import the low resolution version OBJ into Poser and save it as a prop (since it's in a non-standard pose that will be your only choice, it won't be posable). Then you export the normal maps that are generated in ZBrush and apply them in the Material room to get your higher resolution detail.