The process you followed should work fine. You need to make sure the new STEP files are being used by your originally imported Simscape Multibody model, and you should make sure the data file which is produced is also being used. As long as the file names are unchanged and you overwrite the original STEP and data files, then the .slx file will use that new information.
The Simscape Multibody model, once created, refers to the STEP files produced by the export process and has parameter values that refer to MATLAB variables created by the block parameter file. The second export you performed updates both of those files. You would only need to re-import the XML file (and thus create a new .slx file) if you changed the structure of your model, such as adding/removing parts or adding/removing/changing joint connections.
--Steve
