Filter on Update

Hi – is there a way to filter which rows to include when doing an update/merge?

I read the info on importing (https://www.wpallimport.com/tour/advanced-features/) but from what I can see it only applies to imports (ie not merges). Apologies in advance if I have missed it but is there a way to filter on merges?

Background: For some of the data, what is in the WordPress DB will be the most up-to-date, and I don’t want to override it with what is in my external file. I can create a field on the external file to say “don’t merge this row!” but I don’t know where the option is to filter on this field ??