There is probably more than one way to do this, but my approach would be:
1. Export each country's MaxDiff datafile to a .CHO file (this is a space-delimited text file that you can open in Word or Wordpad if you wanted to.
2. Using Word or Wordpad, combine the separate .CHO files into a single .CHO file. Keep track of which respondents are from which country in the sequence of the file.
3. SSI Web cannot read .CHO files (it can only write them), so you'll need to use the standalone CBC/HB estimation software to estimate the HB scores on the combined .CHO file. If you have a CBC license within SSI Web, then you automatically have the right license to install and run CBC/HB standalone software. Download and install CBC/HB standalone software from: http://www.sawtoothsoftware.com/support/downloads/download-cbc-hb
4. After you launch the CBC/HB software, open a new project and point it at the .CHO file, and run the utility estimation, then you need to follow the instructions in the CBC/HB Help Manual, Appendix K "Estimation for MaxDiff Experiments" to do a little bit of rescaling manipulation in Excel to put the scores on the 0-100 probability scale.
[Note: when you estimate scores in CBC/HB, it will look like the last item is "missing" from the score estimation. It is really not missing, since CBC/HB software sets the utility of the last item to 0 and scales the remaining items with respect to this last item assumed to be 0. So, Appendix K will explain to you that you need to insert that last item's reference level of 0 back into the set of scores prior to doing the subsequent rescaling step to put them all on a 0-100 probability scale.]