If you are using FileMaker 11, you may be able to use the OnObjectValidate script trigger to run a script to reformat your data with the missing : before the validation rule on your time field can kick in a display an error message telling you that they are missing.
With older versions that do not have this specific trigger, you'd need to set up a text field for input and a time field that recieves the properly formatted time value via auto-enter calculation or script.
On calculation in the script could look like this:
If ( Length ( Text ) = 6 ;
Left ( Text ; 2 ) & ":" Middle ( Text ; 3 ; 2) & ":" & Right ( Text ; 2 )
or something simllar.
You can also filter out numbers on for the Text:
Filter ( Text ; 0123456789 )
fileer out only numbers before it gets recalculated with colons.