Faby,
if you already have a shape file, why don't you use that for the join? You can do a join with a shape file just as well. Open your existing elevation shape file in your GIS, adjust it as needed, add a field for the K data, export it as a new shape file and use it in your join to set the Ks in FeFlow. That seems a lot easier (and more straight-forward) than using ply files. Either way, you need to manipulate the DBF file, since that's where the data is eventually taken from. For working with shape files you'll need a GIS (I recommend Manifold GIS because it's very powerful, user friendly, and inexpensive!). The DBF is part of your shape file (which is really 3 files: shp, shx, dbf), so once you export your modified shape file with the K data, your dbf will be up-to-date automatically.
Good luck, Chris