The path in the registry key you gave is a value of type REG_EXPAND_SZ. As the Inno Setup documentation for the [Registry] section states there is a way to append elements to those:

On a string, expandsz, or multisz type value, you may use a special constant called {olddata} in this parameter. {olddata} is replaced with the previous data of the registry value. The {olddata} constant can be useful if you need to append a string to an existing value, for example, {olddata};{app}. If the value does not exist or the existing value isn't a string type, the {olddata} constant is silently removed.

So to append to the path a registry section similar to this may be used:

[Registry]
Root: HKLM; Subkey: "SYSTEM\CurrentControlSet\Control\Session Manager\Environment"; \
    ValueType: expandsz; ValueName: "Path"; ValueData: "{olddata};C:\foo"

which would append the "C:\foo" directory to the path.

Unfortunately this would be repeated when you install a second time, which should be fixed as well. A Check parameter with a function coded in Pascal script can be used to check whether the path does indeed need to be expanded:

[Registry]
Root: HKLM; Subkey: "SYSTEM\CurrentControlSet\Control\Session Manager\Environment"; \
    ValueType: expandsz; ValueName: "Path"; ValueData: "{olddata};C:\foo"; \
    Check: NeedsAddPath('C:\foo')

This function reads the original path value and checks whether the given directory is already contained in it. To do so it prepends and appends semicolon chars which are used to separate directories in the path. To account for the fact that the searched for directory may be the first or last element semicolon chars are prepended and appended to the original value as well:

[Code]

function NeedsAddPath(Param: string): boolean;
var
  OrigPath: string;
begin
  if not RegQueryStringValue(HKEY_LOCAL_MACHINE,
    'SYSTEM\CurrentControlSet\Control\Session Manager\Environment',
    'Path', OrigPath)
  then begin
    Result := True;
    exit;
  end;
  { look for the path with leading and trailing semicolon }
  { Pos() returns 0 if not found }
  Result := Pos(';' + Param + ';', ';' + OrigPath + ';') = 0;
end;

Note that you may need to expand constants before you pass them as parameter to the check function, see the documentation for details.

Removing this directory from the path during uninstallation can be done in a similar fashion and is left as an exercise for the reader.