A monster's hit points are derived from their hit dice
From the introduction to the Monster Manual on Hit Points (p. 7):
A monster's hit points are presented both as a die expression and as an average number. [...]
A monster's Constitution modifier also affects the number of hit points it has. Its Constitution modifier is multiplied by the number of Hit Dice it possesses, and the result is added to its hit points.
For example, if a monster has a Constitution of 12 (+1 modifier) and 2d8 Hit Dice, it has 2d8 + 2 hit points (average 11).
The wording of the latter paragraph, and in particular the example, use that a creature's hit points come its hit dice, with a bonus from the constitution modifier (the single number commonly used is the average of hit point representation). Following that logic, adding more dice should increase the creatures hit points. It is worth noting that this is the same logic in the guidelines for creating a monster (ignoring the method which is singly give it a number of hit points) on page 276 of the Dungeon Master's Guide. The creature is assigned a number of hit dice (the size determined by the creature size) which then gives its hit points.
For the specific example, the Nabassu's feature includes (emphasis mine):
Roll those dice, and increase the nabassu's hit points by the numbers rolled.
Which gives an explicit for how the new hit dice affects the number of hit points. Note that it says "increase" and not that it regains hit points, meaning meaning the number of hitpoints should be increased, irrespective of whether any hit points have been lost previous.