I can't think of a way to do it directly without a macro. However, indirectly you can use the following set-up:
enter time as hhmmss in cell a1. Put cell a1 format to text.
In cell B1 enter: =left(a1,2)&":"&mid(a1,3,2)&":"&right(a1,2)
Put b1 cell format to time format hh:mm:ss