The survivorship settings don't perform operations on the values, but select among the existing values. So if the sum was one of the values, using the 'Max' survivorship strategy would do what you wanted.
However, have you thought about update scenarios? How would the sum be differentiated from the existing values that were previously summed? While I don't know all of your requirements, etc., it seems like it may be safer to put the attribute value for the sum in an additional attribute.
------------------------------
Gino Fortunato
Senior Solution Engineer
Reltio
------------------------------