OK.. you got me more curious..
So if I have this bit

  Move 9223372036854771712 to ibResult
  Showln "7ffffffffffff000 = " ibresult " OK"
  Move (ibresult+$fff) to ibResult
  Showln "7fffffffffffffff sum = " ibresult " OK"
then it works:
7ffffffffffff000 = 9223372036854771712 OK
7fffffffffffffff sum = 9223372036854775807 OK
So it looks like the bigint data type itself can do it.

It's still a bug though.