Query regarding 'at end of'

I have to sum a field of my internal table (t_dfkkop-betrw).

The issues is this has to be summed at end of another field (tdfkkop-gpart).

Now this is the third field in my internal table.

So i cannot use 'at end of' properly because the first two fields change before the third field (t_dfkkop-gpart).

I cannot change my internal table as it is direct reference to the database table (and it has to be that way).

Please help..