Use a help column like in the example below:

830 08:30 00:15 8:45
745 07:45 01:15 9:00
1645 16:45 02:15 19:00
00:00 03:15 3:15

columsn A is your input
column B uses this formula:
=LEFT(TEXT(A1;"0000");2)&":"&RIGHT(TEXT(A1;"0000") ;2)
column C is a time field with an increment
column D adds the increment to the time in column B.

Hope this helps

Hans