.dso was the geometry format for .daz scene files (along with .dsv for UV sets and .dsd for morph deltas). On its own I'm not sure it has enough information to be useful, nor am I sure you could load it into DS (possibly via a script). If you ahve the original .daz file too you could load that into the current version of DS (probably, it would depend how old it was) and export as OBJ for Poser.