Adding custom heroes without replacing the default ones.

So, is it necessary to create a mod that adds them to the game data?
And could you provide an algorithm for creating a hero, please?

Unfortunately, that's not my area of expertise... just experiment with it... it's not that complicated (except for heroes that use scripts, but you need to look at the ones in NHF_E - most of them don't use scripts), but it will take a lot of time and patience!