Select merging string only, not all the line in KDiff3

Question

I work on files with long strings and I would like to compare and merge them the way I want after checking all modifications.

For exemple, let's say I have fileA.txt: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.'

Then, a co-worker modifies it in the following way in fileB.txt: 'Lorem ipsum dolor sit amet, consectetur WITH MORE TEXT adipiscing elit, sed do eiusmod tempor incididunt ut labore.'

There is two modifications:

1. adding text 'WITH MORE TEXT'
2. deleting the end of fileA.txt

In KDiff3 when I click on merge, because this is only 1 line, the merging tool only propose fileA or fileB. I would like a file accepting the modification 1 and refusing the 2: 'Lorem ipsum dolor sit amet, consectetur WITH MORE TEXT adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.'

How to do it in KDiff3 ? Or which other free tool ?

This is only a short exemple, I have this on file with thousands of lines. So I need to check all modifications.

Thank you !