Batch tagging IPTC for expedition kit?

[FinnHolt]

Hi all
Hoping someone can point me in the right direction. I've finally got round to photographing my entire supplies cupboard (mostly freeze-dried food, water filters, stoves etc) to keep a proper log of expiry dates. I'm using MP to sort the files on my laptop. The problem is when I grab a bunch of photos, say for the 25-year storage tins, and try to batch add a category like Long Term, it seems to wipe the previous tags regarding the flavour or calories. Is there a proper way to append keywords without deleting the old ones? Dont want to have to start from scratch after spending all weekend on this.
Cheers,

[xnview]

how do you add a category? could you send us a source file?