Unfortunately, I am using SQL 2005, so it's a varchar. Here is the expression. Pardon the spaghetti code...
=(SUM(Cint(Split(Fields!ACDTime.value,":").GetValue(0)))
+ (SUM(Cint(Split(Fields!ACDTime.Value,":").GetValue(1)))
+ Sum(Cint(split(Fields!ACDTime.Value,":").GetValue(2)))\60)\60 ).ToString
+ ":" + ((SUM(Cint(Split(Fields!ACDTime.Value,":").GetValue(1)))
+ Sum(Cint(split(Fields!ACDTime.Value,":").GetValue(2)))\60) Mod 60).ToString
+ ":" + (Sum(Cint(split(Fields!ACDTime.Value,":").GetValue(2))) Mod 60).ToString