An alternative would be to use two columns; one for hours and one for minutes. Then use the following formulae to total the columns assuming the hours are in column A and minutes in column B (assuming a listing of 20 different times to be added)
Total at bottom of minutes column
=SUM(B1:B20)-(ROUNDDOWN((SUM(B1:B20)/60),0)*60)
And total at bottom of hours column =SUM(A1:A20)+(ROUNDDOWN(SUM(B1:B20)/60,0))
Hope that helps
Cheers
Whirls