Adding rows in two dynamic columns

I have two columns in a table that are imported from another table, and
then I have specified dynamic data ranges for them using
Insert..Name...Define and I use OFFSET and COUNTA to select these ranges.
What I need to know is....If I need to SUM each row of the two columns
together, how do I get the SUM column to extend when I add more data to the
other two columns? I have the sum of the two columns using this formula

=SUM(INDEX(SUMDataCLA,ROW(), 1), INDEX(SUMDataCLU, ROW(), 1))

and then I use the FILL DOWN to fill it to the end of the columns, but when
I add more data, I have to use the FILL DOWN again to extend the SUM
formula down. I hope this question is clear!

Thanks,
Justin Etheredge