Custom Separator In Text Field

 I think you would have to do this with two separate fields. One would be at a dead entry field and the other would be a display field.

I think because you're going to display back to the user in your dots separated format, there's a good chance that the users at least some of them might enter the data in that format. So the formula will probably have to strip out any non-alpha numeric characters like a period,  and then format the data to re-introduce the dot separators in the correct locations.