Hi Luc
I don't think you can sum seminar duration out of the box as seminar reports. do not contain the duration as one of the fields.
Adding a custom field of numeric type so that it can be summed is also not possible there is a feature request for this TL-11232
This means to get this to work an export of the custom field string or start and end date would need to imported into a spreadsheet and formulas to sum and convert strings to numbers would need to be created.
regards