Crystal exports data to Excel using the data type of the data in Crystal. Since you're formatting the data as a string in Crystal, it is exported as a string in Excel. So, you can't sum it because it's a string.
How is your data stored? If it's in date or time format, you can put it on the report, right-click on the field and select "Format Field" there will be a tab for formatting the time or the datetime and you can do custom formatting.
If you have to convert it from another format, I would convert it to a datetime and then format the formula to not show the date and show the time in the format you need.
-Dell