Haven't got Excel but an idea occurs...
If possible, try expressing times in angular measure (degrees, minutes and seconds). If Excel will handle angular measure in this format, otherwise remember the conversion from radians to degrees (2 times pi radians is 360 degrees).
These will total up to so many degrees, minutes and seconds. Just convert the degrees portion to hours and that might get you going in the right direction, especially if you are trying to total up logbook hours (which I assume is something like the intended function).
Dunno if this is helpful, but it is a lateral thinking approach which has worked for me in the past on a slightly different application.
