I think it is currently not possible

The iSilo format is undocumented and there is no tool yet that can open it for editing purposes. So you would have to edit them html sources of your downloaded site PRIOR to converting it to iSilo.
I recommend you have a look at
HTTrack which is my favorite web site downloader.
Oh there is one alternative. You can use iSilo for Windows to extract any iSilo documented into a text file. This can be useful if you only have the converted iSilo file but not access to the original source.