Summing a column in excel that is of custom type h:mm

"The [h]:mm:ss format should be another value in the custom list under (in Excel 2003)

Format -> cells -> Number tab

then

category = custom

then scroll down the ""Type"" list until you find

[h]:mm:ss

If it's not there you can type it into the single box under where it says ""Type:"" on the right hand side of the tab."