I want to add monsters on the quest map from the H&M boxes but I don't see a way to add them. Is it possible to do so and how?

The quest vault does not currently have the hero and monster packs as expansions. This is probably because they do not have tiles and don't usually affect quest building. A few suggestions:

1) You can still type the monster in the text. There is no need to add and expansion to use that monster group.

2) If you want to show monster placement on the map, you could use the red X. If you want a [semi-accurate] picture, you can add the CK as an expansion.

Thanks for the suggestion, I guess it's what I'll do then!