Minecraft Command for subsequently changing NBT data (1.14.3)?

I would like that when the player picks up an item (queried by / execute), NBT data will be ADDED WITHOUT REPLACING the item (eg, a book that works like a kind of questlog, so all the quests which one accepts), because if the item is replaced, the old NBT data will be deleted.

You can't change players' nbt tags from players but for entities it would be / data merge entity… And for blocks / data merge block…