Cannot SUM datetime field in Sql Server migrated from Access

I am new here and am new to SQL Server (Express). I used the SSMA tool and it converted Access date/time fields to datatime2. The data contains duration data so there is no need for a date. I want to SUM the field but I need to change to a Time or numeric data type due to 8117# errors when I SUM. SQL Server will not allow a conversion using the designer / ALTER commands as there are always implicit/explicit conversion errors. Thank you for any possible solutions you may have to offer.