The following should get them into Excel time format, assuming the 6
digit number is in cell A1:

=VALUE(LEFT(A1,2)&":"&MID(A1,3,2)&":"&RIGHT(A1,2))

Format as time and 13:30:55. Copy the formula down.

I assume that all numbers are 6 digit, i.e. that you have leading
zeroes for a time like 8:15:00.

Hope this helps.

Pete