Modding

The griffin from NVS is more of a reskin, not an NCF unit.

Units from NCF are fairly easy to convert into Castle units.

For example, there's the Chastener
it has a CreatureVisual file located in \GameMechanics\CreatureVisual\Creatures\Neutrals\Chastener.xdb

If you rename a couple of folders and the file itself so everything looks like this: GameMechanics\CreatureVisual\Creatures\Haven\Renegates\Longbowman.xdb
then the shooter will get the Chastener's model, description, and icons.

But that's only half the job — you also need to rework the map object.
The Chastener is located in MapObjects/Neutral/Chastener.(AdvMapMonsterShared).xdb
The Longbowman is located in MapObjects/Haven/Renegates/Longbowman.(AdvMapMonsterShared).xdb

Besides replacing files, you also need to change one line — at the very bottom of the file there's a line 
    <Creature>CREATURE_227</Creature>
you need to rewrite it as 
    <Creature>CREATURE_LONGBOWMAN</Creature>
and that's it — the unit is replaced.

This method only works for old-school units; it can't be done with Zahar's system.