Modified script with Drive API v3:

Before you use this script, please enable Drive API at Advanced Google services.

Now add the script.

function importXLS(){
  var folderBId = "###"; // Added // Please set the folder ID of "FolderB".
  var files = DriveApp.getFolderById('1hjvNIPgKhp2ZKIC7K2kxvJjfIeEYw4BP').searchFiles('title != "nothing"');

  while(files.hasNext()){
    var xFile = files.next();
    var name = xFile.getName();

    // Check if the file has the MimeType .xslx 
    if (xFile.getMimeType() == "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"){ 
      var ID = xFile.getId();
      var xBlob = xFile.getBlob();
      var newFile = {
        title : name+'_converted',
        parents: [ folderBId ] //  Added
      };
      file = Drive.Files.create(newFile, xBlob, {
        convert: true,
        mimeType: "application/vnd.google-apps.spreadsheet"
      });
      // Drive.Files.remove(ID); // Added // If you want to remove the original XLSX. 
    }
  }
}

References:

• Files: create

• Files: insert

• Files: delete

• MimeTypes