The .hbu file is just a text-only file that may be opened in a text editor like Notepad. It has a global header section, that defines the attributes and levels, and includes labels. But, after that follows respondent data. You can simply copy-and-paste additional respondent data below the existing respondent data, as long as the number of attribute levels remains the same and is properly defined still in the global header. Save that new .hbu file (now with more respondents) and you can upload that to the simulator.