You can use an external command line tool for unzipping your archive, see here for example. Put it in your [Files] section:
[Files]
Source: "UNZIP.EXE"; DestDir: "{tmp}"; Flags: deleteafterinstall
Then call it in your [Run] section, like this:
[Run]
Filename: "{tmp}\UNZIP.EXE"; Parameters: "{tmp}\ZipFile.ZIP -d C:\TargetDir"
(You’ll probably want to take your target directory from a script variable, so there is some more work that needs to be done)