Can .ply file transform to .step file?

PLY is a polygonal model representation, while B-Rep is a boundary representation (usually based on analytical surfaces and curves). STEP format supports tessellated representation, but it is merely useless for something other than just displaying a model in the viewer.

From the other side, reconstruction of B-Rep model from it's polygonal representation is a complex task (sometimes called reverse engineering), which involves surface reconstruction algorithms and other non-trivial steps.