If all the functions indeed return integers, perhaps a workaround like this will work:
if length($myString) < 10
then parseInteger($myString)
else parseInteger(substring($myString, 0, length($myString) - 9)) * 1000000000
+ parseInteger(substring($myString, length($myString) - 9, 9))
Note that the above expression will not parse all negative values correctly and doesn't work for values larger than MaxIntValue * 1000000000
, but you could extend the expression to deal with these cases.